I thought that if you convert your JAA licence to FAA, and get an FAA IR you will abe able to: (a) get an automatic IMC rating from the CAA to fly G reg planes in IMC in the UK FIR and (b) be entitled to exercise the priviliges of your FAA IR in N reg planes anywhere in the world.
If so, this sounds like a sensible approach to getting a proper instrument rating without the roadblocks put in place by the JAA/CAA