I hold an IR(R)/IMC, and am now going for the CB-IR. A couple of points.
- The IR(R)/IMC is only valid in UK airspace, and only up to class D. No "big" airfields, no airways.
- You can carry forward 15 hours of IMC training towards the CB-IR. The rest needs to be done at an ATO, specifically for CB-IR training.
- The theory requirements for a CB-IR, although less than for the IR, are significantly more than the IMC/IR(R). Expect several hundreds of hours of self-study, and at least 10% of this needs to be done in a classroom. Realistically this comes down to half a day classroom-based revision per subject, and you've got seven subjects to get through.
- The flying requirements are significantly higher too. Not only is the navigation accuracy requirement a lot stricter, you also need to do intercepts, holds and a bunch of other stuff.
- As an in-between step you can consider the EIR. The EIR theory requirements are the same as the CB-IR, and the EIR gives you airways privileges. Together with the IMC for approaches and departures, this gives you almost the same privileges as the CB-IR. And any training towards the EIR will count towards the CB-IR as well.
I highly recommend doing the IMC first. As said, 15 hours of this counts towards the CB-IR anyway, and it gives you a good feel for what IFR flying is about. It also gives you considerable skills that you can use as a "get out of jail for free" card when flying VFR. And if you plan things properly, you can use the IMC to go touring in the UK. But if you want to perform serious touring, including mainland Europe, go for the EIR and eventually the CB-IR.