The IRS provides guidelines for choosing a tax preparer. These include making sure the preparer has a PTIN and to check with the US BBB for questionable history. The IRS also has information about understanding tax preparer credentials. Attorneys and CPAs are licensed by US states so it seems unlikely that I will be able to find one in the UK. What types of certifications should I look for in a UK based preparer for US income tax returns?
The issue is actually reliability of the preparer and their reputation and their ability to prepare your return legally and with little or no pain on the tax front.
Having said that if you Google "Tax Preparation UK US" the first thing that does come up in that list is the link from US Embassy in London which gives you a list of providers of Tax Preparation services in the UK some of which like Grant Thornton are well known some others not so much.
In addition to that list Google provides other companies that provide the same services as the preparers on the list above.
As far as evaluation is concerned the best way to evaluate a Tax Preparer from my past experience is chatting with other clients if it is possible. The issue with people doing one's taxes at least in the US is whether or not they meet your individual goals and expectations but knowing that is only possible from either someone you trust or from gathering information from current or former clients which have similar goals, which takes time.