Your main priority is to bank 800k per person into your Thai bank accounts with proof of transfers from abroad. You can then apply for your Non Imm O visas based on being 50 or over.
Get the Non Imm O evisa from London, much easier than getting it in Thailand, it'll solve your flight problem, save you a trip to Immigration, make it easier to open a bank account and it'll work out cheaper than an extention and visa in Thailand.
Once you get the 'affirmation to marry' translated and stamped by the MFA you can get married straight away. You wont get the Non Imm O based on marriage untill you're actually married.