For the sim card just get a tourist sim and after a while try to change it to postpaid. AIS seems good enough. If u want he 12 month extension of stay its 800kb, easy to get if over 50. I will get criticized for saying this but: 1) Your state pension will get frozen when u move to TH. 2) The tax laws in Thailand have changed now you are taxable if u are here 180 days, all uk pensions except civil service pensions are taxable. 3) Review all your assets before you become resident. watch carls vids to help ypu get upto speed with taxes. Or maybe you are going for the LRT?
I think they are making it up as they go along, even by accident they will change the rules, i have not heard anything about 90 day reporting and I guarantee they have not informed immigration on this issue and off course income tax. I see also many issues like setting up branch offices without even knowing it and even Thai sales tax, for those operating companies oversea the issue of corporation tax on dividends. For those who do not pay tax I would not be suprised to hesr when they get vack home, they get a tax bill there.