because with a simple PPL you must navigate by ground reference
Reference please.
It's rubbish of course. It is 100% legal for a PPL to navigate en-route using VOR/DME/NDB/GPS or dead reckoning, in any combination. The ANO prescribes the equipment that must be CARRIED, only.
There could be an implicit assumption that one lands visually, reasonable enough because a plain PPL does not have instrument privileges. But even that is untrue because a plain PPL can, in the UK anyway, fly an ILS in VFR conditions.
It is possible for the ANO to be amended to state that a GPS is not permitted, or that every pilot must carry a rubber duck with a magnet on the back of it, floating in a bucket of water (the bucket would need CAA approval). But the ANO has only just been amended and the new version has only just come out, so I doubt they will be doing another version for a while. And the latest ANO does not mention GPS at all.
Think about it. How can a law be drafted to force a pilot (single pilot, no passengers) to navigate using a particular technique? It can't, of course.
You can do PPL instruction with these views but you would have difficulty flying a decent distance with them.
As for calling the CAA, please supply the name of the contact who believes GPS is somehow unauthorised.
I always dive into these stupid pro-GPS anti-GPS threads because if left uncorrected they spread a lot of damaging disinformation which benefits nobody.