Download LASORs from the UK CAA. It has lots of applicable information. Search PPRuNe for a link to or go direct to
Civil Aviation Authority Home Page and search there.
There is a shortcut to a JAR ATPL if you have lots of command time on heavies. The catch is that it's a UK peculiarity of the implementation JAR rules so may not be recognised by other States.
Otherwise do the theory via correspondence. You'll still have to attend a multi-week (6?)course as well but it's a lot shorter than the full time theory course! No minimum training for the test for the initial issue of a professional licence but you'll have to do a minimum of 15 hours IR training for that one.
The test will have to be in a 2 pilot aircraft. If you have less than 500 hours multi crew time then you'll have to do a Multi-Crew Cooperation course (2 weeks, no exam - just acceptable attendance) before the test. Not sure if the test can be done in a sim
That's the precis version. LASORs is a good resource. Even though it's UK specific publication a lot of the information applies throughout JAR-land. You should shop around the JAR members for advantageous rates.