One can assume CAS busts are caused by nav errors, or by map reading errors.
I find the latter incredible because IMHO the CAA 1:500k VFR charts are by far the clearest of
Jepp VFR/GPS (lousy choice of colours, occassional ambiguity)
French SIA (poor terrain info)
Swiss ICAO (stupid mixture of ft and m in elevations)
FAA sectionals (one needs to know the implied airspace types)
Only the French 1:500k IGN charts compare for clarity with CAA but they omit stuff above 5500ft, making them IMO useless for serious flying.
The CAA charts are exemplary for clarity and lack of ambiguity. A pilot from Mars could use them right away. Helped by the fact that one really can just about "fly anywhere" in the UK; Class G is plentiful and there is no French-style military airspace all over the place.
So, what causes nav errors? Candidates must include:
1. Lack of currency: Present-day GA scene is too decrepit and so attracts too few people with funding adequate for flying. So, most PPLs fly too little, 10-20hrs/year?
2. Peer pressure to use old fashioned methods: Too many old farts in GA telling everyone that stopwatch+compass is the only real way to do it. This was fine 50 years ago when there was little CAS to bust, and is OK for a pilot who has been flying around the UK for past 30 years - even if he flies very little nowadays. It's not OK for a recent low-currency PPL.
3. Poorly equipped aircraft: most planes flying are old rental spamcans in which very little works. Obviously this argument is on a hiding to nothing if you believe in the old fashioned methods but it also scuppers any attempt to modernise.
It's tempting to blame the often poor training but I can't see how anything else (a full inclusion of GPS in the PPL being the obvious thing to do) can be included without increasing the minimum hours from 45, and few people want that. It would also necessitate mandating panel mounted GPS installation in training aircraft, which the industry would really hate.
Instrument-capable pilots routinely navigate GPS/VOR/ADF/DME when when flying VFR but IMHO it would not be productive to get PPLs to use VOR/DME routinely because, at the very low levels enjoyed by many pilots, one cannot receive the navaids over much of the UK.
A full inclusion of GPS is the way to go, but it will never happen.