IF you received a FAA certificate issued on the strength of your UK license, you will have to go through the Cert of Authenticity process.This process involves the UK CAA who have to confirm to the FAA that your UK license is valid and current.
Entirely true; ******** corrected SoCal is right *******
If you have traded your UK license for a JAR license the number will be different, in that case you have to go through the "validation" process.
http://www.faa.gov/licenses_certific..._verification/
You should be in the FAA airman database:
https://amsrvs.registry.faa.gov/airmeninquiry/