Retirement visa/ extension requires b800k to remain in Thai account for 3 months after initial application, then deposit can be reduced to b400k until 3 months before renewal when it must go back up to b800k. Banks have started making there own rules for issuing the IO required statements and certificates.
Seems you want a marriage visa which has different requirements than a retirement visa discussed here so far. Afaiu marriage visa capital deposit is b400k.
UK govt does not issue pension certificate statement required for b65k monthly so that route is not open to you.
All IO offices vary. Based on my dealings with CM airport,
B800k must be transferred in to Thai account from abroad 1 month before retirement extension application and remain in there for 2 months after when it can drop to b400k until 3 months before renewal is required. Accurate dates and amounts are essential.
Thai banks seem to be moving goalposts on issue of certificates and statements required by IO as well as initial opening of accounts. Reportedly if the required amount has not been in Bangkok bank for 4 months prior to certificate/statement request the bank will freeze the account until 4 months has lapsed.
As AP + BT advice but I would suggest you get a Thai mobile number ASAP (easy to do at most 7-11). If you don't have a lease/ condo already you may be able to pay a few months in advance for your hotel and supply that to the bank and IO as proof of address. (works in CM, other IO may be different). Means you don't have to rush the choice of where to live.
Firstly commiserations on being in this situation. I may be wrong as I am not in the exact same situation but AFAIK for retirement visa the b800k has to be in the Thai account for 3 months after the award then it can go down to b400k until 2 months before renewal when it has to go back up to b800k. Exact dates and amounts are crucial. I'm sure someone else can advise on how to do the b65k monthly on the renewal. It may get you out of the hole to take a loan of b400k for the first 3 months? Good luck.
p.s. Bangkok bank are reportedly getting awkward about the certification of deposits required for immigration to do renewal which (reportedly) is making agents more awkward.
It appears your understanding of English and Thai is limited. Best for you to find a lawyer or agent that can communicate in your mother tongue for advice.