englishal
To my knowledge (no Class A airspace) if you file an IFR flight plan it just goes to the destination plus the regional service (e.g. London Info)
There is no controlled airspace clearance implicit; you still have to ask for it as you go along.
I may have missed something but I don't see how having a full IR would change this.
I agree with your comments about 15 hours; for most people it isn't enough to do what you describe, but equal in my view is the problem with suitable aircraft; the great majority of planes available for hire, or with shares available, are basically junk as far as IFR flight goes. But the instructor who has just taken £3000 off you for your IMC Rating, and who is hoping you will self fly hire his school's plane afterwards, isn't very likely going to tell you this. I know a few people who are doing their IMC Rating and not one of them is aware they won't be able to get their hands on a suitable plane afterwards in the location in question.