Something to be aware of if you stay in thailand for greater than 6 months in any one year (doesn't have to be all in one visit) then you become a tax resident in thailand. As you are over 50 you could apply for a non o retirement visa that gives you 90 days in thailand which can be extended by a year and then yearly if you meet some financial requirements ( 800k baht in a thai bank account), you can apply for this on the thai embassy visa website through the London embassy before you travel. You can also cuttently get a 60 day visa exemption entry into thailand (although they are considering reducing this to 30 days) and extend this in thailand at a local immigration office for 30 days for 1900 baht, giving you 90 days. The caution with this is that over time they may not allow you to enter as you are using it too often. You can also apply for a 60 day tourist visa through the evisa online system and again extend 30 days inside of thailand. There are a number of other options but they start to require more funds or complications like the LTR visa the non oa retirement visa and the elite visa
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.