I understand I cannot fly commercially on said license in the UK
Why not? Plenty of people do, just needs to be an N-reg (or Isle of Man, if we're being pedantic) aircraft. The bizjet industry would be on its arse if you couldn't.
I would like to be able to fly under instruments when I return to blighty
Again, in an N-reg you can do whatever you like. The CAA will give you an IMC rating based on your FAA IR which confers pretty much the same privileges as an IR outside Class A airspace.
can I show up at any old flying club, get checked out and go fly?
Yes. If your club tell you any different, they should brush up on their air law.