Proof of flights from UK to thailand: I would simply get a "fake"/flight reservation and I'd cancel it after, usually and I magine them don't check when arriving, or do they?
I mean, things can change, you could go to Vietnam after applying or don't go back because of some reason, so you enter from Vietnam
I'm doing same but actually they changed the progress to apply for a tr visa. It's an evisa now, nur sure about UK citizen but German citizen only apply online for tr single or multi entry from last year on.