Hi John Saw, when weighing up my options earlier this year. I considered returning to UK, checked FCO site all that I needed could be acheived by post with them.
In the end had Monthly income verified by Embassy Bangkok.
Believe you will need a UK address for postal then documentation forwarded to you.
Site saved so will look it up and repost here. Or add comment to Thai Visa site.
Jamie having made a phone call you have effectively have 5 options.
1. Apply for an Education visa, this would have to be done through an agent. 70,000Bht annually.
2. Apply for an ELITE 5 Year VISA cost 500,000Bht.
3. Marry your young lady apply for extention Thai Wife.
To conduct this you will need to do a affirmation of freedom to marry at the British embassy Bangkok, have your passport verified as well. Total cost for these approx 4000Bht.
In addition prove you have 40,000Bht income per month or annual equivalent,
This done via post to British Embassy cost approx 2000Bht.
Have Translations of passport and Affirmmation completed 2000Bht