Wrong datum is a very common error, especially if flying club a/c. As many people are neither trained in using a GPS unit nor take the time to self-study, they tend to fiddle - and change the datum to something useless. Happened to me once flying a club a/c, had the GPS on w/o really looking (t'was a local bimble in an area I know by heart) and, approaching a VRP, discsovered it was about 5 miles out on the GPS. Checked after landing and sure enough, the datum was set to something weird. Reset it and hey, presto! all was well again. Just shows that it's idiotic NOT to train the use of GPS during the PPL.
Anyway, back to the OP. FWIW, one thing that helped me early in my flying was to not only look at the charts but also at road maps. In fact, I always had one with me in the a/c. These maps often display roads and topographical features in a better way than aeronautical charts. Plus, you're probably used to looking at them and finding you way round using them! Obviously not for primary navigation, but helps. Another thing to remember is that you will be looking at any feature at an angle, while the map depicts lakes, towns, etc as seen from straight above. Unless you do your PPL in the Space Shuttle, an unlikely view. In general the UK's a bitch for visual nav anyway, as there are relatively few distinctive features - it all sort of looks alike (at least in the South East), even the Tescos all have the same sign
It therefore helps to try to pick
isolated landmarks, like power stations, cement factories and the like. Initially, make sure you have something to refer to every 10 miles or so, as you progress you'll increase the distance between these landmarks. Venturing further afield, always try to find some long(ish) feature running perpendicular to your track, such as a highway, river, ridgeline, etc. that can act as a sort of 'catchline'. I'd be careful using lakes as landmarks, especially if they are not islolated as they can look very different at an angle than as depicted on the charts. Also bear in mind, that if said lake is a reservoir, it can change shape and size considerably.
HTH