The way you do is is to drop out of the IFR route by descent well before the IAF or the start of vectoring. You are never going to fly to the overhead at 3000', that's either silly for you or it's not going to work for ATC.
That is interesting.
I dont fully grasp your aversion to joining at our near the IAF.
I used Southampton as an example because from experience they will often vector you near the IAF (Now I agree it is not exactly overhear the IAF) before turning you out bound for the ILS join. If you know the cloudbase is above your EIR minimium, but within some hundreds of feet of it) I dont see why you would want to make up a descent beforehand and I dont see why it would cause ATC any issues at most smaller airports (my comments were not referring to the major airports handling a huge volume of CAT). That seems to me a lot more sensible than working away with the charts and making up your own descent perhaps many miles away from the airport while leaving the protection of CAS and having less assurance of the actual cloudbase where you expect to pop through.
I must be missing something?