Sadly the schedule doesn't really allow for a day trip anymore but if you could do an overnight LHR-INV and go and see the reindeer herd in Aviemore which isn't far from the train station. Before retiring somewhere to sample some whisky. Think Inverness will have a Xmas market too.

Looking at it HBO flights in December pre schools breaking up are about £65.

ETA: one way from INV-LHR looks about £28 if RFS is LHR-INV