I think we're talking about tourism. All flights from Ubon go to Bangkok. Krabi has flights to KL and Singapore. Hardly a comparison. Ubon isn't on most tourists "must do" list!
Yeah but with all due respect Ubon is an outback airport that once had a flight to Laos so it grabbed the "international" tag to pump up its importance! Krabi is the sixth busiest airport in Thailand
UK has a very bad DTA with Thailand. Fortunately us Aussies and the Americans as well, have a brilliant DTA which excludes pensions. Can't help you with this one!
By post it can sometimes take ages to get the report back. Provided you sent it 15 days before the due date, don't worry about it. I used to do it all the time by post, and one time got the report back after eight weeks