Originally Posted by IO540
If the IAP is outside CAS, ATC have no authority.
Incorrect.
By definition, under UK regs to have an IAP you have to (on the whole, there are rare exceptions) have ATC. BY that reasoning if ATC say you can't, or are not cleared to fly an instrument procedure then you can't. To do so would mean you would possibly be eroding separation from an aircraft lawfully allowed to fly the approach.
Yet another reason why all IAP's in the UK should be protected by at least Class 'E' airspace, and as such should only be flown iaw with an IFR clearance. (Like the rest of the world).