Not everyone needs help with American tax returns. Tax software such as TurboTax and TaxAct can cope with some circumstances. The IRS in London have a free walk in service at the Embassy.
Here is a list of Enrolled Agents, which might help:
https://portal.naeacentral.org/webportal/buyersguide/professionalsearch.aspx?Token=- scroll down and choose United Kingdom.
Golden Rules for choosing a tax preparer:
1. QUALIFICATIONS
Do not be shy! Ask if they are members of professional bodies in both the US and UK and if so, which ones and for how long.
2. COST
Ask what they charge!
3. COMFORT
Pick one who is dual US/UK qualified and who you feel you can get on with. This is personal choice so call a few firms before deciding...