Doing the carb upgrade is easy. Getting your hands on a #20E sounds like it's pretty difficult in the UK. One of the other guys had called like 40 hobby shops in the UK something. If you have a freind of someone that has an old OS Max .21 RG, the carb is the one you need (20e). That's how I got mine. I lucked out and a guy here had one and sold it to me for $20. I think he still has one laying around but shipping to the UK is prohibative from what I hear.

-RobH