FAA 61.75 certificate revalidation
Having just gone through this process myself, the procedure is:
Firstly, you need to apply to the UK CAA for a licence validation, and this is just a case of filling in a form that you can download from the CAA website. Send it off to them with a fee of £44 and they will then send confirmation to the FAA when requested. There's no requirement to make a personal visit to their office.
At the same time, you need to send or fax a 'Verification of Foreign Licence' form to the FAA. Details can be found on the FAA website. They say to allow up to 90 days for the verification to be issued, but mine only took about 3 weeks.
Once you have the verification from the FAA ( I got an email and then this was followed up with a verification letter in the post), you need to arrange an appointment at any Flight Standards District Office (FSDO) in the US and bring along your licence, medical certificate, passport and verification letter. If everything's OK, they will issue a Temporary Airman Certificate there and then, and at no cost! This will give you full privileges while you wait for the full certificate, which I believe is now a plastic card, to be sent to you in the post in due course.
All in all, it wasn't that complicated but I did grudge parting with 44 quid to the CAA.
Ian