Would be interested to know what you find out. The US Embassy does have an FAA London Office but am not sure what it is for as there are no 'Inspectors' available.
I got my plastic one back in 2004, a couple of $s and it arrived in 10 days.
The new ones should also have "English Proficient" added to them and there was a big argument about getting this done for UK based pilots as apart from the UK based DPEs who charge a large fee, you'd have to visit the US ("with ID in person") to satisfy them. I've done this once and it is not fun.
Alex.