0 baht to put into an account? If you were in an OA then you were able to show evidence of equivalent in your home country. If you transfer to an O you'll have to season
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0 in a Thai bank until the extension is granted and not let it drop below
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0 at all and make sure it's back to
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0 a few months before the next extension. Your issue is having used an agent. You are stuck in that loop of renewal with them after using them. Going alone, or moving province (as you have) basically means starting from scratch. The plus side is, now you are doing this all yourself through the official channels you shouldn't have this issue again, as long as you keep to the rules of money in your account. You may wish to do some research on the difference between the O and O-A visas and the different requirements though.
if you can find decent insurance at a good price then maybe just get the cheapest authorised one with the biggest excess and just see it as a visa surcharge. If you've no intention of using it then I guess it doesn't matter if you don't fully disclose your history
you'll see a lot of people grumble on here about the o-a because of the health insurance part but it worked well for me but I'd only just hit 50 so the premiums weren't too high. Opening a bank account was dead easy as well as I had the long term visa in hand. Many people seem stuck in a catch 22 of wanting to open an account to get the long term visa but get refused because they don't have a long term visa. Bangkok Bank actually have a branch in London. You may even be able to get the account open before you leave, although I've not seen anyone post on here they've done such a thing but their website does mention savings accounts and current accounts
If you don't want to tie the cash up and are happy to pay for the year insurance get the o-a (which, with a multi entry you can get almost 2 years out of, with a timely border bounce), open your account and then start doing the monthly transfer. When your o-a expires you then have the proof required to to get the o on monthly payments. Criminal check and medical check are simple and will set you back maybe £100. Only issue I guess is of you're worried the criminal check won't come back clean
did i say he'd be blacklisted? No, I didn't. He is more than welcome to pay an overstay fee at the airport if he wishes. Will the stress of dealing with that and any consequences of being stopped by the police in that short period of time between visa expiring and departure be something the OP is fine with? Only he can say. Financial saving of paying a couple of days overstay vs spending a few minutes getting a tourist visa to cover his entire stay that will guarantee no possible issues with boarding in the first place, to me, isn't worth it. Seems like a lot if risks for a £10 saving