The legal reference is Article 103 of the ANO 2005. As I understand it, this rules originated many years ago when two aircraft either collided or got very close when trying to fly an IAP without ATC. Not all States have the requirement for an approach control service. Note that the legislation doesn't require an aerodrome control service but, it seems, the CAA is now introducing such a regulatory requirement for GNSS-type approaches if what you say is correct.
For some reason the UK/CAA has trouble distinguishing between the function of an IAP, to get from in/above cloud etc. to a point where a visual landing can be completed, and and the function of ATC which is to provide separation between aircraft (whatever they are doing).