Apply for the non-O visa based on married to Thai as soon as possible.
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The Thai embassy in London is very efficient and you should have it in probably less than a week.
It will be valid to enter Thailand for 90 days and when you enter you'll receive 90 days permission to stay.
Once you arrive in Thailand try to open a bank account as quickly as possible. You need the 400,000 to be in your account for 2 full months when you apply for the 1-year extension so you have less than a month to get it opened and funded and still be able to apply for extension.