you need to get a non O visa before travelling to Thailand online from the Thai Embassy in your country. It used to be possible to come without a visa and open a bank account, but that's not possible now.
If you avoid the non OA visa, you don't need criminal checks and medical insurance.
It's more complicated than getting a non O visa before you come to Thailand, as it's virtually impossible to open a bank account without a long term visa.
Seems like you know the answer already, but the legal option is obviously the answer.
You could do it legally and transfer 65,000 baht each month in addition, so that the second year you can withdraw your money and put it back into your savings account in the UK.
I found Santander to be the worst of my accounts when I came to Thailand, they froze my card very quickly. Luckily I most of my money was in other accounts.
As Lynnette stated (and other people have said it to me previously) Nationwide seem to be the most lenient, so maybe try them.