Others have answered on where to fly.

For your short duration trip, it may not be worthwhile to get a US certificate to fly N registered gliders in the US. Just fly a 2 seat glider with a US pilot qualified to act as pilot in command.

To get a US certificate based on your UK certificates, you'll need to file paperwork with the US Flight Standards District Office (FSDO) for the KS area in advance of your trip. They will verify your paperwork with the UK. Then you present yourself at the FSDO with your paperwork, certificate and passport ID and you can be issued a US pilot certificate at the private pilot level. Then complete a US Flight Review with any US instructor and you are good to solo or act as PIC in a US registered glider.

I'm not sure of the time line required to file the paperwork. But a search on FAA.gov may provide the information.

BillT