Flying from Bangkok to Glasgow, I had to go through this searching of my cabin bag etc at Heathrow as well as Swampy.

On the return flight I did not need to do this at all, straight of the Glasgow flight, then waiting for my flight back to Bangkok.

Anyone know why I should have had to go through this at Heathrow before boarding my flight to Glasgow, and not on the return flight to Bangkok?

At a guess I'd say the UK authorities have greater confidence in Glasgow's security checks than Bangkok's (or indeed any other foreign land).

Its even stricter as UNO tells: it has to do,with (sorry or not-brexit or scot?) EU-regulations: airports can/may trust searches done in other EU-airports, but not those from elsewhere. Thats also why f.e. flying EY (=etihad) one has to do with that whole new rigmarole again on each transit, after all UAE does not trust anyone else for that. Thus would have been the same if you changed in AMS or FRA in stead of LHR.

Probably when they realise also this yet another thing those brits have to think about  brexit-follow-up rules.