Why not go the commonly used FAA IR route and get your IMC on the back of that. The FAA IR seems to be fairly stable, reasonable to achieve in both cost and requirements plus it gives you an additional avenue of use if you so choose
Indeed; the only issue is that for the IMCR you need a valid UK PPL (plus the CAA medical, etc).
You also need an FAA IR
IPC in the past 2 years; the CAA reportedly does not issue the IMCR on the basis of the rolling FAA IR currency.
Another thing is that the FAA IR is a reasonable stepping stone to a JAA IR. You "just" have to sit the PPL subset of the JAA ATPL ground exams (about 10 of the 14) and do the checkride. The UK CAA also requires 15hrs of flight training in this case but some/most other JAA states don't.
If you have a FAA PPL/IR and get the FAA CPL, then you immediately have an FAA CPL/IR. This is what I did. There is no IR re-test. The FAA CPL is a VFR-only thing. There would be an IR re-test for ME, of course.
It's only the ATPL which has additional and tighter instrument requirements but that's not too hard to do either - just one written exam. The catch with an ATPL is that you need 1500hrs and this has to include ~ 100hrs at night, etc.
However, there is no practically relevant context in which an FAA CPL/ATPL is of any use in European airspace. Every European country has protectionist legislation banning commercial ops on foreign licenses (or foreign reg aircraft).
AFAIK the only use of an FAA CPL in UK airspace is that you can be a paid company pilot, contractually required to fly the company's plane on company business.