What documents do I need to provide for my DVT visa application while already in Thailand?

The visa costs over £300. It sounds like you don’t need one as you get stamped in for 60 days at a time. They probably know when you got stamped into Thailand from passport number.

It seems like you can not send any additional documents until you've gone back home plus bought tickets for your next flight back to Thailand.

The day before is wild tho lmao

From your email that you received from Thai Embassy in London requesting additional documentation, it seems either:

1.) they suspect or know that you submitted a Thai E-Visa application in Thailand = asking about departure flights exit out of Thailand and departure exit stamps by Thai immigration....

or

2.) you had submitted a Thai E-Visa application to the Thai Embassy in London, but they suspect or know that you are no longer in the UK and asking about departing flights exit out of the UK....

If applied through Evisa you must be in the UK and provide your actual ticket into Thailand from the UK .

So, send them a copy of your BKK-UK boarding pass if you have it, and copy of your exit stamp from Thailand. Did you fly from Thailand to the UK to apply, and then leave for Thailand?

So are they expecting/requiring you to be in UK before they issue the DTV?

You should have stayed in the UK UNTIL they act on the DTV visa application.

What do you mean “new flights” ?

Do you have to show departure ticket for DTV visa from London ?