Just a bit of feedback.
Submitted UK Passport application at Chiang Mai VFS today.
A couple of things......
For a UK adult renewal passport, a countersignatory IS NOT REQUIRED. So no need to get one of the 2 photo's countersigned or fill out that section on the application form. Which saves a lot of hassle!
However, if you do fill out the countersignatory section, (which you don't need to as above), you require a copy of the countersignatory's passport as well and one of the photo's countersigned.
You also need proof of address in Thailand, I used my Thai driving licence. They need it in colour and a copy of the front and back. My address is in Thai and English on the back of my Thai driving licence.
So regarding the application form, it's really only the first page that gets completed, passport number on page 2 and a signature at the end!
You keep your passport, which can be used for domestic flights BUT obviously not for international flights.
All above confirmed as of today at VFS Chiang Mai.