I opened an account with SCB in 1999 on a VOA. Unfortunately after I left Thailand and my funds in this account had been withdrawn by Thai, the bank closed the account. I still have my old passbook & debit card. When I go to Thailand later this year, depending on which visa I get from my home country, I'll go to an SCB branch and ask if I can open another account. Can but try.
There's only two non- immigrant visas you can apply for in the UK. They are a non-immigrant 'O' multiple entry 12 month visa, which if with this visa you have to do a visa run every ninety days, but on your last ninety day of this visa you can get a thre month extension, giving you fifteen months altogether. To qualify for this visa, according to the e-visa portal, you have to have a family member that is a Thai national living in Thsiland. The other non-immigrant is a ninety day stay one entry visa, which anyone over fifty years of age with a UK bank account showing I think 200,000 THB equivalent balance. This visa will let you open a Thai bank account and can be extended by twelve months at an immigration office in Thailand as long as you have one of the three monetary methods of qualification. The other visa you can apply for while your in the UK is an A-O retirement visa, which I believe you do not need to show any financial requirements, but must have a twelve month health insurance policy that is recognised by Thai Government, ie., immigration. You can take out insurance with a UK insurer for the initial year, but after that for your next visa extension, you are required to have insurance from a Thai immigration recognised insurer. This info., is as I have researched.
I had a similar thing donkeys years ago. I went into an immigration office and told them I was too ill to travel and my visa was about to expire. They said they could give me a fifteen day extension for 1,900 baht. Do it yourself, agents just rip you off. Try, nothing to lose.
For anyone who doesn't have 800,000 baht, but can do a combination of money in Thai account plus monthly transfers from outside Thailand, like pension, that accumulates to the required 800,000 baht.