SC
It is currently illegal to use a GPS as primary form of navigation in the UK.
The above is complete and utter RUBBISH. RUBBISH. Quote the passage from the LAW, the ANO please. (I will save you time, there isn't one).
The best you will find is a repeat of your (totally unsupported) assertion in some handout from the CAA ("safety" sense leaflet #25 is a likely culprit) written by some old fart who has never actually flown anywhere and whose exposure to GPS is from looking at them in the locked glass cabinet at the local camping/hiking shop.
SF's requirement is a 100% legitimate thing to have, for both VFR and IFR flight and especially for IFR.
One day, people with the backward views will have left aviation and the whole scene will then be able to move on and modernise, and attract some normal modern people, not the desperate-to-fly people who make up the bulk of new PPL recruits today. The problem is that the average G-reg GA fleet age is about 27 years and it will all be scrapped long before attitudes change sufficiently.