The US Embassy list remains the only list I know of that has so many dual US/UK qualified tax advisers listed who are in or around London, which was the original question on this thread. There are however perhaps roughly 40 firms in the London area today preparing US taxes, but I have never seen a complete directory yet, which I think is a shame.
I agree 100% that there are of course many great CPAs and EAs in the US who can handle US tax issues and may also be able to prepare UK returns and provide joint UK/US tax consulting advice as well. I agree entirely with nyclon that a tax preparation firm such as taxesforexpats, greenback or expatriatetaxreturns.com may suit many people much better than a more local firm, it depends of course on exactly what you are looking for.