Haven't been there yet, but looking into it at the moment. You have three options:
- UK IMC rating - 15 hours minimum of instruction, will allow you to fly G-registered aircraft in IMC, but only in the UK and with some other restrictions
- FAA IR - minimum 40 hours instruction. Lets you fly an N-registered aircraft in IMC, which is great if you have an N-registered aircraft. But also gives you a "free" IMC rating, so arguably better than doing the IMC rating... as long as you don't mind going to the US to do 40 hours of training
- JAR IR - the 45 hours of training isn't really a problem, the ground-school is. Not worth the hassle unless you're going commercial, or you have a specific need (or desire) to fly a G-registered aircraft in IMC outside the UK
Whatever you decide, good luck!
FFF
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Edit to reply to FD's point about getting an IMC rating first to test the water... The instruction time that you need to get the IMC rating can be deducted from the time required to get an FAA IR (reducing it to around 20-25 hours minimum, depending how quickly you get the IMC). However, it
can't be used towards the JAR IR. So if your ultimate aim is to get a JAR IR, there's no point doing the IMC first, unless you want to end up paying for 15 extra hours.
And, while we're on the subject of doing more than one of these options, you can convert an FAA IR (or any ICAO IR) to a JAR IR with just 15 hours minimum of dual. But I think you still have to do the tedious ground-school if you use this route.