Golden Rules for choosing a great tax professional when in the UK:
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 dually US/UK qualified (so you know that the tax preparer is regulated in both the US and the UK) and who you feel you can get on with. This is a personal choice so call a few firms before deciding...