You'll need a JAA CPL to work in the UK, the UK CPL is no longer up for grabs.
If you go to the UK CAA website
here you need to download a book called "LASORS" which contains all of the licencing rules. That does contain all the relevant details on converting a foreign CPL to a JAA one via the UK CAA.
I'm a bit too specialist to be an expert on employment as a CPL in the UK, but from what I do know it comes down to experience - a bare CPL won't get you anywhere, but a CPL with 1000 hrs doing air-taxi in Islanders might stand you a chance.
G