You may want to look into using an enrolled agent - there's a locator here: https://portal.naeacentral.org/webpo...ch.aspx?Token=

Enrolled agents are certified by the IRS for doing US taxes, but there are several in the UK and they may have further qualifications in UK taxes. Depending on how elaborate your tax situation is, you may get as good or better tax assistance from an EA, and at a significantly lower price. (An online friend of mine is an enrolled agent in San Francisco and I know she does very good work.)
Cheers,
Bev