in Europe I must inform ATC and request a "practice" approach if it's not real IMC/IFR. I don't know where/whether this is in the rules, but home base certainly insists on ILSes being "booked" in advance if it's VMC.
It's nonsense. A suitably rated pilot is entitled to fly IFR the whole way. VMC has nothing to do with it. Most commercial traffic is required to. A well planned IFR flight will be in VMC much of the time anyway
Anyway, let's say the weather is poor or marginal and then improves. You aren't required to cancel IFR just because it's improved.
There is no such thing as "IMC/IFR" except that flight in IMC has to be IFR. An IR/IMCR can fly IFR anytime he chooses. Abroad, or in Class A, it just gets more regulated (mandatory flight plan, IFR clearance).
As for booking an ILS, that may be the individual airfield rule, for a "training" flight, if they have half a dozen twins doing IR training in the circuit. A lot of airfields around Europe have restrictions, often a total ban, on "training" flights. But a pilot can still do a fully IFR flight and the destination has to fit him in. That's the ATC job. He may have to hold, etc.
For example Cranfield has a lot of IR training traffic and tries to discourage people going in there IFR if the conditions are OK for a VFR arrival. But you can just do it and they can't stop you. Especially if they have the flight plan.