I highly recommend calling the FSDO and confirming they have their copy.
I second that. Working with the FAA re obtaining a US "airman certificate" on the basis of a foreign license is not a particularly difficult process; HOWEVER, you need to take the time and trouble to ensure that (1) the FSDO has received authorization from Oklahoma City and (2) you have a pre-arranged appointment. Before you leave the UK, take care of this via e-mail, and then telephone the FSDO's contact person to double check everything.
Once you get to the FSDO, it's a painless process ... you just have to get all of your ducks in a row beforehand.