To apply for the Non O in Thailand, you'd need to deposit 800K in a Thai bank account, which defeats your object. Obtain Non O in the UK. Deposit 400K in your Thai bank account. When seasoned for 2 months apply for the 1 year extension based on mariage. At the same time as transferring the 400K, start monthly transfers of 40/65K to suit. The following year, you have the options of changing the reason for the extension to retirement and use the 65K monthly transfers to meet the financial requirements. Alternatively, only transfer 40K per month and continue with the extension based on marriage. I'm always tickled pink by those that claim extensions based on marriage are akin to jumping over hurdles and through hoops. The extra documentation is minimal. Copy of marriage certificate, copy of name change certificate (if applicable, photos taken in and around the house, all of which can be done from home. The only extra trip is to any local Amphoe for an updated KR2. Trip to the bank is the same regardless of the reason for the extension. A deciding factor could be how near or far you reside in relation to your Immigration office. Even extensions based on retirement are not all completed the same day and require a revisit for the extension stamp as some offices. Everything is local for me, but if I lived 60-90 minutes from my local IO, then I'd consider an extension based on retirement.
