I was planning on doing the same, ended up going for an mp5sd as I have a few m4's already.
My plan was to buy a specna arms 416 for about £200 and use the gearbox shell that came with it. There didn't seem to be many receivers for sale in the uk.
My inferno dmr is a krytac receiver with a 3rd party gearbox shell and it took a while to get it to play nice, always best to use the shell that's designed for the receiver.