Suggestion, It might be an idea to ring the relevant FAA office before you try to turn up, and explain what you want, they may be able to arrange a visit for you, or how you get around the problem. When I have phoned them up before they have been very helpful, whether you ring from England or the US.
The main office is at Oklahoma City:
Flight Standards Service
Airman Certificate Branch,
AFS-760
Federal Aviation Administration
PO Box 25082
Oklahoma City
OK 73125
Tel
001) (405) 954-1738
I have access to most of the other FAA office phone numbers if required.
Reference UK to FAA office, you will only be issued with the FAA equivalent of a PPL, whatever UK licence you hold. If you want an FAA CPL/ATPL you have to complete all the relevant exams. Just in case I got peoples hopes up too much, that you could convert a UK CPL/ATPL to a straight FAA CPL/ATPL.