Hi,
As a UK national, you don’t have to provide a residence permit but a prove of your residence inside your kingdom. An electric or telephone bill is enough.
As for the garantie letter from your spouse, there’s no templates but a simple signed letter from her stating that you’re still married and that she supports your visa application is enough. For the proof of the residence in Thailand, copy of her house book (Tabien baan) or yellow book is acceptable.
I provided these documents and got the visa within 10 days in France.