Lots can happen or not between now and your trip, don't worry as you get closer to your date I.e. within a month if the rules have changed then just apply for a 60 day single entry tourist visa on the e-visa system, typically takes around a working week to get, then you'll have no issues with your flights or entering thailand
you cannot get a non oa visa inside of thailand, only a non o visa. Although to get a non o visa inside of thailand requires a thai bank account which is very difficult if you don't arrive with a non o from the uk
Airbnb isn't illegal in thailand. It is only illegal if you use/stay in Airbnb for less than a month this is so they don't compete with the typically short stay hotel vacations. So if you are staying in an Airbnb for a month or more it is legal.