As an alternative to the 'onward flight' consider a Booking.com booking (subsequently cancelled) for a hotel in Siem Reap and say that you are meeting friends in Surin and will depart Thailand at the land border at Chong Chom/Osmach.
Probably.
There has never been any health requirement for the application for a UK Visa. Covid entry rules applied in the UK at one point - but they have all been removed.
Do you have a WHO reference for that.
I generally follow rules, that is the society in which I was brought up in.
I don't necessarily agree with all those rules.
I don't know, but I would doubt it as Skyscanner are a travel fare aggregator website and travel metasearch engine.
https://dpogroup.com/everything-you-need-to-know-about-skyscanner/
I would argue that the European Summer holiday break is a minor factor - this is low season for a reason; less people travel to Thailand.
What the proposal does impact on is the planning for 'winter' holidays i.e. Thailand high season. I know several people who are now likely to book something.
UK Visas are taking about 4 weeks.
The most recent one I saw was submitted to UKVI on 17 May and the 'passport dispatched' email was sent on 8 June (22 days, virtually back to the stndard 15 working day processing).