Part P does not control socket height this is BS7671:2008 which says they should not be placed where likely to be damaged. And Part M which says where you may have visitors using the sockets they should be placed where the disabled can reach them this varies according to building type.
Mounting sockets where car doors will no hit them or where front or back of car is unlikely to damage them is main point.
Garages are really for cars. And mounting in a position where with the car in place they are inaccessible or likely for damage is clearly wrong even if you never put a car in the garage.
So mounting high so as not to impede assess and egress when car is in place would make sense.
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