The issue of recency is a probable cause in its own right regardless of aircraft, conditions or navigation technique. However, I would suggest that one would have to be very un-recent to let a 100 knoter get away from you to the extent that one wonders into the Heathrow CTR. Recency in my view, is something that merely exacerbates an existing problem, I don't believe it can be the route cause for the issue at large.
I think the factors I gave are contributory but I agree they are not the whole answer.
In any given year, countless thousands of GA pilots weave their way about that area, using all kinds of nav, ranging from map reading to GPS. The "true locals" just "know" where they are, without a map - I know because I've flown with a few of them.
And only a few hundred, say 1%, bust some piece of CAS.
It would be useful to find out just what this 1% did, which the other 99% or so didn't do.
I think many other pilots get lost, both PPLs and students, but they get away with it. One student I trained with got totally lost on his QXC. The instructors were biting their nails for the max possible fuel endurance time; about 5 hours and then got very worried. Eventually he phoned in... he ended up wondering around Norfolk or Suffolk for hours and eventually saw a runway and landed on it.
I suspect those 1% were just unlucky. Low currency means high cockpit workload and mistakes are more likely. I've flown 220 instead of 200 a few times, or set the wrong target altitude/level on the AP. Anybody can do it, even professional pilots do it all the time.
There are IMHO two approaches to this: just accept it as something humans will do no matter what, or drastically overhaul the training process. One could argue this both ways.
The FAA has a good approach: if you turn up on the checkride with a GPS+AP equipped plane, you need to show you know how to use the equipment. This ensures that all recent PPLs know these basic things, while avoiding the FAA getting blamed for making the PPL more expensive. Whereas in the UK we just pretend this kit doesn't exist, and then moan that some pilots don't know how to program the GPS. Americans learn dead reckoning too (PPL and CPL) but it's quite difficult today to come out with a PPL without having learnt how to load up a panel mounted GPS.