IMC rating: 15 hours minimum, plus test, plus any extra time it takes you. Let's call it 20 hours.
Multi-engine rating: 7 hours minimum, plus test, plus any extra time. Let's call it 10 hours.
That gives you a total of 30 hours. You'll need to do quite a bit more hour-building than that, unless it's taken you an unusually long time to get your PPL.
You are basically correct, by the way. The IMC rating needs to be done at a UK-approved school (it's a UK rating, not a JAR rating). The multi rating needs to be at a JAR school, although I'm sure it's possible to do an ICAO multi rating and convert it (but I don't know the requirements for doing this).
As well as this, though, you will also have to do a long cross country, which doesn't need to be done with any JAR approval, and you should aim to get a night qualification, which also doesn't need any JAR approval. So I'll stick by what I said, which is that if I were to do it again, I'd do my hour-building somewhere "unusual" (and maybe return to the UK for an IMC rating).
At the end of the day, though, it's really not all that important - it purely comes down to your own personal preference of where you'd like to fly, and how much variety you'd like to have. If you've got your heart set on the US for hour-building, then don't let me talk you out of it - I did my own hour-building in the US, and I had a fantastic time.
FFF
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