FAA/JAR Conversion
The cheapest, and probably best place to do your conversion is in UK. They are all geared up for these conversions (In Scandinavia it is very expensive and they may want you to fly Cessna's again because they don't accept your current rating !!)
First you will have to apply for the conversion and give them all your details/experience and they will write back what you need to do.
Basicly, if you have more than 3000 hours on 30.000kg/above with 500 as Capt you can do 2 exams only and then the Skill test on your type with the UK CAA. However, the license will be restricted to UK registered AC only (however, many EU companies will accept it with this restriction).
If you don't have the above experience and/or want a full unrestricted UK JAR ATPL License..there is only one way to go..all 14 written exams and that would include 2x2 weeks on f.ex Oxford Aviation in UK for the written exams (I can recommend them) After completing the written exams (you have 18 month to do so after first attempt) You arrange the skill test with the UK CAA.
The full unrestricted license can be done in 3 month (if you are not working, lucky and bloody smart), else i would think 6-8 month..with alot of selfstudy..and alot of money
Good Luck