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.
I also am going to be in thailand in April but for this visit I have a 60 day tourist visa which only took 3 days to get approved as I didn't want to risk my visit
you need to show 80k US dollars passive income (there are other ways if you read the LTR requirements) to be eligible to apply for an LTR the DTV is much simpler