Anyone used a UK-based tax filing fim for US taxes?

I am really struggling to get to grips with my US taxes -- I've somehow managed to get through it for the last few years, but am fairly sure I have made all kinds of mistakes/omissions, and I just want to straighten it all out and get it done right, once and for all. (Possibly including going back over previous years and making amends.)

Has anyone used a UK-based firm to file US taxes? If so, how was the experience, and how much did you pay?

The short version: I 'm afraid I can't help.

The long version .....
I did contact a UK company about doing my US/UK tax returns for this year.   It was a small outfit but they seemed knowledgeable and friendly.  However when they sent me a formal invitation the fees came to many thousands of pounds, so I politely declined.   I've done my US return with my old tax preparer in the US (IRS certified) the two of us researching and comparing notes in areas that he was less familiar with.   I think we've made a pretty good job of it together in good faith.   We'll see what the IRS thinks.

Unless you have very significant income that can't easily be either excluded using foreign earned income exclusion, or offset using foreign tax credits you may find you pay very hefty fees simply to check your paperwork (in my case it would have been more than 10% of my total US/UK tax bill in fees !!!!!)

It seems the US based outfits (Greenback tax returns, and the company that sponsors this board) have much more reasonable headline rates than the UK outfit I contacted.   With modern communication (cheap phones, emailing etc) I don't see why you'd necessarily need a UK based adviser.   I wish you the best of luck.

Most UK based individuals who do not feel confident handling their own tax returns will want to check that any proposed tax adviser is qualified with at least one professional accounting or tax professional body in the UK as well as in the United States so that they know that the adviser is adequately regulated in the UK and handles every day many of the complexities of the foreign aspects of the United States tax system.  Indeed, more broadly, for most UK based individuals there are often considerable advantages in having a UK based adviser who can jointly advise on both US and UK tax issues and (possibly) handle both sets of tax returns in the UK.

These include:
1.   The client and the adviser being located in the same time zone.
2.   Client documents and workpapers being held outside of the United States; which many people perceive as providing additional protection in the event of IRS audits.
3.   Advisers outside of the European Union are unable to offer the protections to clients provided by the Third European Money Laundering Directive, which requires all tax professionals throughout the EU to be regulated and supervised for anti-money laundering protection purposes.  Here in the UK it is a criminal offence to offer tax advice unless the adviser is supervised. Quite unlike within the EU, there is no requirement for mandatory regulation of tax advisers in most of the United States. 
4.   The fact that although there are roughly one million paid tax professionals within the United States, that there are naturally only very few amongst these who understand enough about the US reporting of foreign based taxpayers.
5.   Most tax advisers outside of the UK are not bound by the UK ethical standards that govern UK qualified advisers; which set out fundamental principles of integrity, objectivity, competence and care, confidentiality and behaviour (http://www.tax.org.uk/professional-standards/professional-rules/professional-conduct-relation-taxation).
6.   The opportunity in the unlikely event that things ever go wrong to get issues addressed through a supervisory body that is closer to the client than an adviser located several thousand miles away.