I'm a CPA myself (living in France) and I have found that it's pretty difficult to find a single accountant anywhere to handle both US and UK (or any other country) taxes. The best placed are the big public accounting firms, because at least those guys have associates back in the US to confer with. But, as you have noticed, their prices are outrageous.

You might try asking around some of the US expat groups in the London area. Many times they have a member or sponsor who does US and UK taxes - either a tax attorney, a tax accountant or an enrolled agent.
Cheers,
Bev