It's 3000hrs in a/c 30000kgs or greater half of which has to be PIC or P1, flying scheduled international routes or similar. At the end of it all as someone mentioned you'll get a G registered ATPL for UK a/c only.
I think you need air law + human perf in addition to the rt theory.
I'm not sure what you fly but I'm an CRJ Captain here in the states, the -700 meets the weight requirement but the -200 doesn't even though it's the same type rating.
After investigating all possibilities, I decided to sit all 14 exams then do a ME CPL IR as the easiest and most useful option. I did look at getting it done via an ATPL skills test in the RJ observed by a CAA Flt Ops type but quickly abandoned the idea when I tried to get a JAA TRTO to do it + figured out that the CAA would charge me several thousand pounds just to watch my sim check with the possibility of paying it all over again if I partialled.
I'd get your medical done first if I were you, it's definitely not a given, despite you presumably holding another 1st class somewhere else.