Is there a requirement for the 500k baht to be in my account for a month when applying for a visa from London?

The general consensus from the comments is that there is no specific requirement for the 500,000 baht to be maintained in your account for a full month when applying for a visa from London. Instead, embassies are primarily interested in your current bank balance. Some users have noted that London only requires a closing balance without seasoning, meaning the funds can be transferred right before applying.