You are pretty much right, yes. An IR qualifies you to fly 'in circumstances requiring flight under IFR' (my italics). The latter means it qualifies you to fly in IMC, and also in class A, since class A is IFR only.
An IMC rating (this won't come up in the exams which are JAR only, and don't mention UK only ratings) qualifies you to fly in circumstances requiring flight under IFR, in class D and below. So it qualifies you to fly in IMC, and to fly IFR in class D, since you don't need a rating to fly in class D VFR.
Confusingly, in the UK the IR and IMC ratings also reduce the VFR minima, to the ICAO ones you will learn anyway for your exam. In the UK a PPL is limited to 3k vis and 'in sight of surface.' But having the IR/IMCR means you can fly VFR on top, for example.
I'm sure I've forgotten something which someone will come along and point out, but that's the gist of it...
Tim