I'm afraid the cut off is 30 tonnes. Then your "JAR" licence is restricted to G reg aircraft. (ie UK)
If you have 500 hours in multi pilot operations (or more, as you do) then all you need to do is a check ride with a CAA examiner and a TRE (Type rating examiner) on the aircraft you have the time on.
Also, you'll have to do the medical at Gatwick + the 14 ATPL exams.
For further info, check out LASORS
Pay very close attention to ATPL Section G info G1.5 pages 4&5. (You'll need Acrobat reader).
Go to Standards Doc 31
for where to study and fly.
To find out about
JAR/FAR 23 & JAR/FAR 25 types
have a look here.
It's a bit of a read but well worth taking the time to do so.