The accounting market continues its robust expansion, valued at £38.5 billion in 2025, with sustained growth driven by increasing demand for specialised international tax expertise. Yet many accounting firms face a critical challenge: while globalisation creates unprecedented opportunities for cross-border services, the complexity of maintaining dual-qualified expertise across the US and UK systems often forces firms to refer valuable clients elsewhere.
At Optimise Accountants, we bridge this expertise gap through comprehensive continuing professional development and white-label tax services that enable firms to confidently handle complex international tax matters. Founded in 2003 by Simon Misiewicz (FCCA, ATT, EA, MBA), our team combines decades of cross-border experience with proven training methodologies that help accounting firms expand their service offerings while meeting strict professional development requirements.
The Growing Demand: UK-US Accountants & Advisors
The modern accounting landscape presents both opportunities and challenges for firms seeking to serve internationally mobile clients. With advisory services comprising 45.1% of market value in the UK accounting sector, the demand for specialised international tax knowledge has never been higher. However, maintaining current expertise across multiple jurisdictions requires significant investment in continuing education and professional development.
These requirements reflect the increasing sophistication of international tax compliance, where expertise in ITIN applications, FIRPTA withholding, W-8BEN-E treaty planning, and expat tax returns has become essential for serving globally mobile clients effectively.
White-Label & Outsource Services: Seamless Client Support
Recognition of expertise limitations should not result in lost clients or reduced service quality. Our white-label services enable firms to maintain client relationships while delivering specialist international support under their own brand identity. This approach addresses a common industry challenge: while outsourcing international tax work may seem like an obvious solution, many firms hesitate due to concerns about protecting client relationships and maintaining control over service quality.
Protecting Client Relationships Through Branded Excellence
Our white-label approach ensures that all client communications, deliverables, and ongoing support appear to originate directly from your firm. Clients receive the same high-quality service experience they expect from your organisation, while your firm benefits from access to specialised expertise that would be prohibitively expensive to develop internally.
This model proves particularly valuable for complex cases involving dual-residency scenarios, international business restructuring, or multi-jurisdictional compliance requirements. Rather than referring these opportunities elsewhere, firms can retain the client relationship while delivering expert-level service through our invisible collaboration structure.
Flexible Service Delivery Models
Recognising that different firms have distinct needs and client bases, we provide flexible engagement models tailored to your specific requirements. Pay-per-case arrangements provide cost-effective access to specialist expertise for occasional international tax matters, while annual retainer agreements offer predictable costs and priority access for firms with regular cross-border requirements.
For firms serving significant numbers of internationally mobile clients, our embedded model creates the appearance of internal expertise while maintaining cost efficiency. This approach enables smaller firms to compete effectively with larger organisations that may have dedicated international tax departments.
Common Pitfalls in International Tax Work. How We Fix Them
| Pitfall / Challenge | How Optimise Accountants Fixes It |
|---|---|
| Double Taxation – Clients are hit in both the US & UK because treaty relief isn’t claimed properly. | We apply US–UK treaty rules and file Form 1116 (US Foreign Tax Credit) + SA106 FTCR (UK) to prevent double taxation. |
| Missed Filing Deadlines – Late FBAR, 5471, 3520, 8621 can trigger $10k+ penalties per form. | We use deadline tracking systems and SLA-driven processes to ensure every filing is on time. |
| PFIC Misclassification – UK ETFs/ISAs treated as PFICs face punitive US tax unless reported via Form 8621. | We run PFIC analysis, make Mark-to-Market or QEF elections, and use treaty-friendly investment wrappers. |
| Share Options & RSUs – UK PAYE vs US W-2 sourcing conflicts create double charge & compliance issues. | We align HMRC ERS rules with US wage sourcing and apply treaty allocation under Art. 15. |
| Pension Rollovers – 401k → SIPP → Roth IRA transfers are often taxed twice if timing or reporting is wrong. | We apply Treaty Art. 17 + IRS Rev. Proc. guidance to structure rollovers tax efficiently. |
| Residency Rules – Errors using SA109 or Form 1040NR cause residency mismatches and enquiry risks. | We coordinate the UK Statutory Residence Test with US residency rules and apply treaty tiebreakers. |
| Data Security & GDPR – Client data sent abroad risks GDPR breaches. | We use a GDPR-compliant secure portal with ISO27001-certified hosting for all files. |
| Currency Conversion Errors – Wrong FX rates distort US/UK calculations. | We consistently apply official IRS average annual rates and HMRC spot rates. |
| Non-Dom Reform 2025 – UK regime changes impact US tax treatment of trusts & investments. | We provide proactive tax planning around rebasing, trust reporting & transitional reliefs. |
| Mismatched Years – US year = Calendar; UK = April–April → income overlaps & double counting. | We reconcile periods with pro-rata allocations and treaty-aligned schedules. |
Implementation and Getting Started
The successful integration of international capabilities requires careful planning and a phased implementation. We work closely with partner firms to develop customised approaches that align with their existing client base, growth objectives, and operational constraints.
Initial engagement typically begins with assessment of current client needs and identification of opportunities for service expansion. This analysis helps prioritise training investments and service development activities, ensuring maximum impact from professional development initiatives.
For firms seeking immediate capability enhancement, our white-label services provide instant access to specialist expertise while internal teams develop competency through our training programs. This dual approach enables firms to capitalise on current opportunities while developing long-term capabilities for sustained growth.
The combination of structured CPD training and flexible white-label support provides a comprehensive solution for firms committed to developing international expertise. Whether your goal is basic compliance capability or sophisticated advisory services, our programs provide the foundation for sustainable growth in this expanding market segment.
Ready to expand your firm’s international capabilities? Contact us to discuss customised CPD training programs and white-label service options that align with your specific requirements and growth objectives.
Why Simon Misiewicz Is the Expert Accountant’s Trust for Cross-Border Tax Support
Simon Misiewicz isn’t just another tax professional—he’s one of the few people qualified on both sides of the Atlantic to help accountants handle even the most complex US–UK cross-border tax cases.
- Dual US–UK Tax Credentials:
Simon is an Enrolled Agent (EA) with the IRS and Certified Acceptance Agent (CAA), enabling him to handle US tax filings, ITINs, and IRS representations. On the UK side, he’s FCCA (Fellow Chartered Certified Accountant) and ATT (Association of Taxation Technicians) qualified, bringing deep UK tax knowledge under HMRC rules. - 20+ Years’ Experience:
Simon has worked with hundreds of US and UK tax professionals—from small accountancy firms to large practices—helping them deliver compliant, efficient, and profitable cross-border tax solutions. - Specialist in Cross-Border Tax Compliance:
He regularly handles cases involving US Forms 1040, FBAR, 5471, 8938, PFIC 8621 filings, UK SA106, SA109, foreign tax credits, pension transfers, and double tax treaty positions. - White-Label Expertise:
Simon built Optimise Accountants to give firms the ability to outsource or white-label complex tax cases so they can:- Keep clients in-house
- Avoid losing fees to competitors
- Access IRS + HMRC technical expertise without hiring internally
- Trusted Trainer & Speaker:
Simon delivers CPD (UK) and CE (US) accredited tax training, helping accountants learn while also offering done-for-you outsourcing for cases needing immediate attention.
