The easiest way is to have a total time of +3,000 hours with +1,500 hours in command of aircraft 30 tonne or more in primarily international or equivalent operations. However, that restricts you to G reg aircraft.
To answer your question you should go to LASORS on the CAA (UK) SRG site and sift through those. It's not an easy read but it should answer your questions.
The "easiest" way does depend on your hours and types flown.
An unofficial guide, details MUST be checked.
**in all of the below cases, 14 JAA ATPL subjects must be done**
**CPL training JAA style, some flying required.**
**IR training, JAA style. Looking at 15 hours min, some can be in a synthetic trainer**
**JAA subjects cover CPL and IR subjects, so no need to duplicate**
**JAA medical must be done, for the UK, this needs to be done at Gatwick**
Aus CPL with CIR and ATPL subjects completed. = JAA CPL with IR with ATPL subjects completed. Need to do MCC. (also known as "frozen ATPL")
Aus ATPL with CIR, no multi crew time = JAA CPL with IR with ATPL subjects completed. Need to do MCC.
Aus ATPL with CIR, 500 hours Bandeirante {FO or command} (ie not JAR 25 aircraft) in RPT ops = JAA ATPL, need letter from company explaining time was done multi crew. Need to do a type rating (JAR 25 type) with a TRTO and do the LST observed by the CAA. Don't want to do that and fly a piston. JAA CPL + IR, MCC exempt so long as you have the letter.
Aus ATPL with CIR, 500 hours Dash 8 or 737 {FO or command} (ie JAR 25 compliant) = JAA ATPL. Need only to do enough sim stuff to get you up to speed and pass an LST observed by the CAA.
Source documents.
LASORS CAA SRG
JARs
www.jaa.nl