I have always thought that an O-A takes time and expense compared to plan A. do you have to attend in person at Embassy in London ? As I live North East uk, I think Hull i easier if I can get away with one way flight, if not I noticed that a BKK to Vietnam flight was obtainable for £20 ish.
Just a bit concerned that the flight outbound might be fussy, haven't booked flight yet. I have previously been asked about VISA's on previous one way flights, but had a retirement Extension at time so wasn't a problem.
the answer is fairly irrelevant as he will be stamped into the 27th or 28th, leaves on 28th, so 1st day is 500 Baht Exempt, not that i would encourage overstay. PS google anything and get 3 answers to same question