You should be able to open a bank account yourself, even if you are on a tourist visa or visa exempt stamp. All banks have different requirements. All branches have different requirements. Even within a branch two staff members may give very different answers. Find a branch of the bank you want (I use Bangkok Bank in Doi Saket), in an area with lots of foreigners. Go to the bank with your passport and a friend who holds an account at that branch. Have 100,000 thb to deposit. Should work but many banks/branches will also require a certificate of residence from Immigration. But instead, you could offer a letter from your hotel/ hostel/ Airbnb/ friends house stating that you stay there. Doi Saket Bangkok will work with that.
There is no extension for your visa on arrival. If you apply for and pay for an extention you will be denyed and given a stamp with gives you 7 gays to leave the Kingdom.
You will have the difficult task to prove to the UK authorities that your wife has exceelent reasojs to exit UK at the end of her visa. eg, good job in Thailand (letter from employer stating the period of her leave and the lenght of her employment), own house in Thailand (or lomg term rental agreement), need to return to look after child, parent or close relative, proof of ealier travel showing abiding by visa conditions. Difficult for a Thai wife to get UK/US/Australian visa. All have similar requirements.
Provided that you can obtain the required medical certificate from a doctor/,edical clinix/hospital and the residency certificate from your local Immigration office.
If you are not a tax resident of Australia then you do not receive the tax free threshold and pay a minimum of 44 cents in the $ tax. Much more than in Thailand. And the tax authorities in Thailand do not know how much you are taxed in Australia, So I expect that Thailand will tax you at 15% or at 0.