I took mine at Farnborough before I went to the US. I used the ASA online testing system which provides the required sign-off for the exams.
Doing the exams in the UK saved time and stress when I was in the US so it worked well for me. You can also get the TSA fingerprinting done at Farnborough which also saves time in the US. That said, looking at your location it's a bit of a trek from there down to Farnborough.
A lot depends on how long you are planning to go to the US for - if your trip there is going to be several months then a few days for the exams is neither here nor there. If you are only going for a few weeks then you will want to get your ducks in a row before you go.