What do you mean I would be wasting my money for private flying?
You'd be wasting your money getting a JAA IR for private flying. The IMC provides adequate IFR privileges while in the UK, which you get for "free" as the holder of an FAA IR. A JAA IR is an expensive rating to keep in Europe, unlike the FAA counterpart, so if you plan on doing relatively few hrs IFR in Europe, you might as well forget it.
The CPL is only 20 hours. Seems one of the easier ratings
Don't forget all the requirements to meet as well as the 20 hrs (like 250hrs TT and the rest). If you want a ME CPL, you'll need probably at least 20hrs in a twin minimum before you get it.
You won't gain anything (cost wise or experience wise) by doing the CPL in Europe from scratch if you have the opportunity to get the FAA one and convert.