Ever noticed the compass is Magnetic? Mag Hdg plus Drift =Mag Track! Why would you want to use TRUE?
Because everything the Met says is TRUE and, with a VFR chart, one tends to align one's protractor to the lat/long lines so the two crucial bits of information you have are in TRUE until converted. Why convert two things for every leg, when simply creating a TRUE heading means you only have to do one? As mentioned, 3 degrees variation in the UK isn't exactly going to put you overhead LHR unless you got it all very wrong in the first place.
Granted, once you are comfortable with diversion planning, then it is much easier just to write everything you need on the chart directly and adjust in the air as necessary. But I doubt most will be comfortable with that until they have a few hours solo doing it the rigourous 'proper' way.