I converted my FAA ATP to a CAA (UK) ATPL. I was given alot of credit due to my time in two-crew, transport aircraft (in excess of 3000 TT and 1500 PIC in Group II aircraft). I took 2 written exams: Air Law and Human Performance and Limitations, a very comprehensive medical exam, some fixed base sim time to practice the procedures of the company that I would be flying for, a warm-up sim period, then the IR/PC that was given over a three day period with the CAA observing.
I agree with some of the other posters that alot depends on prior experience as to the amount of credit you are given toward the conversion. Also, I believe my situation was simplified somewhat by my employer, in that they had significant experience dealing with the CAA in the conversion process.