I think RM still send a lot by air, either dedicated mail flights (Titan Airways a/c?) or as cargo on commercial flights. Probably depends not only on how far it is going (e.g. Scotland) but also how fast it has to get there (e.g. overnight vs. second class).
This would seem logical, but in practice, because RM have fixed contracts for flights, if they have a 10t aircraft going and only 7t of express mail that night, they will drop in 3t of second class etc; likely saves some road haulage cost. This is one of the causes of seemingly random delivery dates, with items sometimes appearing to have overtaken others posted earlier.