Each JAA country has their own tests. The UK CAA has several testing centers, one of them is in Florida (which is actually just in a conference room at a hotel in Orlando each month.

If you hold any ICAO ATPL, then you don't have to be enrolled in an ATPL theory course to take the exams, aka you can self certify. If you don't hold an ATPL, (you must hold at least a PPL to take the JAA ATPL exams), you must be enrolled in an approved course and have a minimum level of attendance. If you do distance learning, you must finish the whole course and then sit at least 10% of the required hours in residence.

For more info on UK approved testing centers overseas, CAA International Ltd

If you want to see all the schools approved by the UK CAA, look here:
Standards Document 31 Version 107: Organisations Conducting CAA and JAR-FCL Approved Courses of Flight and Ground Training - Interim Document | Publications | About the CAA

There are different providers but many of the schools in the US providing ATPL theory training use someone else's materials under a license agreement, typically through distance learning.