I've spent a long time searching for a good, affordable service. Let me know if you find one.
I tried going with a UK adviser, and spoke to about 6 firms but I had bad experiences with them all - either non-responsive, or gave me bad advice, and in one case even led me on as a client before dropping contact bizarrely. They were all UK Chartered Accountants w/ US tax experience, but no actual CPA/EA in the company - they all outsource the actual filing of your taxes to a US based firm.
Eventually I gave up trying to find a UK based firm, and talked to Taxes For Expats and Greenback Tax - which are both US based firms employing only CPAs that market heavily to expats. Both seemed to be quite competent and gave me timely advice. I went with GB just because it was a bit cheaper at the time, and they really helped me out with a complex situation.
Unfortunately both Taxes For Expats and Greenback are quite expensive in my opinion for any non-trivial return, as they charge a fee PER form. Also unlike a traditional accountancy practice, you can't just ask them questions on the fly - they charge you per tax consultation. As someone self-employed this all adds up very quickly. So while they seem quite competent it is so expensive. I'm not sure what I can do about this, apart from finding a US tax practice who will charge more reasonable fees. But I've had no luck on this front, unfortunately.
So at this point, my recommendation would be to try either Taxes for Expats or Greenback. They seem quite competent, it's just expensive.