I was in the same boat Rob. I've gone down the 12 month multi entry Non Imm O visa from Savannakhet based on marriage to a Thai. That seemed to be the only option for me as I'm not at the ripe old retirement age yet. If your already in Udon (and married to a Thai) you could sort that out whilst your home this time and skip the trip to UK. The banked money/monthly deposit for subsequent extensions for this visa is also lower than the retirement visa option I believe.
Someone asked this question a few weeks ago and I think it came around that they could also used banked money as they were in the same position as you.
I asked at Udon Immigration when I did my last TM30 if I have to do this everytime I leave and enter Thailand (as I frequently do for work) and they said "no, only when you change permanent address"
However bear in mind things change quickly and the guy next to the one I spoke to may have a different opinion TIT
Just incase anyone is interested before admin close comments. You can apply from abroad via post and require the usual documents for a Non Imm O based on marriage. They will accept printed online banking statements for proof of funds (website shows 3 months worth) and the turn around has been noted to be pretty fast. There we go, all questions from the OP answered.