The link which matspart3 provides shows clear evidence that it
is currently legal, and accepted by the CAA:
Flying an IAP not approved by the CAA will be prohibited.
(The implication being that it is not currently prohibited.)
These measures will affect... some individuals using non-notified IAPs.
(The implication being that some individuals use non-notified IAPs.)
It is also clear that if the proposed changes go ahead, this activity would become illegal:
Proposed changes are shown underlined... as appropriate.
...
90-(2) A person shall not cause or permit any instrument approach procedure to be established or used to fly to any aerodrome in the United Kingdom otherwise than under and in accordance with an approval granted by the CAA to the person in charge of the aerodrome.
The author also makes it clear that there are safety implications with the current, legal situation:
There will be an improvement in safety because all IAPs and associated navigation aids (navaids) will be designed and properly approved for the purpose.
FFF
--------------