Port Strobe - there's a thread running in Private Flying which asks exactly the same question. It wanders off-topic a little bit, but it does answer the question befoer it wanders off topic - read that for a full answer.
The short version is that the IMC Rating is only valid in the UK, as you already know, and it's only valid in Class D airspace or lower. There are recommended limits for instrument approaches which you should not go below - broadly, the absolute lowest for a precision approach is 500' and for non-precision is 600', but it's more complex than that.
As for "what I'm not going to be taught that an IR would teach me", you should really be taught everything except airways for an IMC, except you won't have as much practice, and therefore you won't be capable of flying it so accurately.
(There are some things like holds that aren't strictly speaking on the IMC syllabus. But if you're going to fly an instrument approach and ATC need you to hold, you're going to have to know how to do it - which is why every IMC instructor or examiner I've spoken to says that they require IMC students to be able to fly holds, although they're not at all bothered if they're not accurate as long as they're safe.)
FFF
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