You will have been issued with a US airman's certificate back in 2001, but it will need to be replaced with the new credit card style.
You can do an
FAA airman search online to see that they have a record of your original airman's certificate.
Your local FSDO can arrange the change from paper to plastic.
It will still be valid if you still have the UK licence that was listed on the certificate. The number change only applied to JAR licences changing to EASA. UK national licences were not affected.
So the minimum that you will need to be legal to fly again will be a medical (FAA class 3 or UK class 2) and a current flight review with an instructor.
The amount of training to get you back up to speed is basically up to you and does not require you to go through the security process as you already have the licence.