What are the steps for transferring a Thai marriage visa to a new passport while in the UK?

I have just had the same issue happen to me. I left the UK with a non o visa, I had 10 months on my passport. Marriage visa required 6 months on my passport. By the time I got married and a bank account took me under the 6 months by 7 days. I had to get a new passport, and new visa. I am going tomorrow to get my marriage visa approved.

Oh I had to start everything again from day one. Bank statements tomorrow and then off to Immigration for my marriage visa approved My problem was that it took 2 months to open a bank account. Even after I had opened it and deposited the money. They came out to my house to check that I actually stayed there before they would actually open my account.

Yes

If you apply in uk obviously before your passport expiry date would the 10 years start from that date you apply or the expiry date in July ,so July 37

It's got nothing to do with the UK what stamps you have to enter Thailand. It's a very simple process and you are worrying without reason. Simply get a new passport in the UK. Travel to Thailand with BOTH passports. Present at immigration showing your visa extension and re entry permit and you will be stamped in as normal up to the end of your extension, in your new passport. When convenient to you, visit your local immigration office to transfer the visa extension stamps to your new passport, this is normally free of charge.

Would your last extension only be up until the passport expiry date ?

Assuming you have a re-entry permit you travel back with both passports and get the stamps transferred at your local immigration office after you return to Thailand