First Ive heard of any such restriction.
Have you checked the Royal Mail website ?
VISIT AND READ HERE
https://www.royalmail.com/sending/international/country-guides/thailand
I would copy them anyway, the application will be reviewed at the VFS, if they want them, you have them. They have full copy and translation services at VFS so you shouldnt have any problems.
Thai passport is not relevant to UK passport application.
You are renewing it is NOT a new application.
Renewal requires copies of ALL pages of passport.
Application submitted via VFS in Bangkok or Chiang Mai.
Four words answers this from a practical point of view " luck of the draw" !
What is stipulated and what happens on a daily basis are not the same.
People seem to order(vitamins etc) frok Herbalife all the time without issue.