Can I move my 400k baht from my Thai bank account when applying for a marriage visa from the UK?

Keep in mind that there are substantial insurance requirements when purchasing a Non-O visa based on marriage in the UK from a thai consulate (and they are not required to get the visa issued inside thailand)

I would recommend instead that you get a single entry tourist visa OR come in on a 30 day visa exempt entry and then go to your immigration office and apply for the 90 day Non-O visa for 2000baht.

You already have the funds in a thai bank account in your name, so you should be able to do it from a 30 day visa exempt entry OR a tourist visa entry

nope, you will still need to bank 400K baht in a thai bank for 2 months before you apply for a year extension based on marriage inside the country