I have seen some references, including from the TAT, that you must be at least 21 days in your departure country prior to travelling to Thailand for the Sandbox process.  Did this requirement make it to the final cut as I don't see it mentioned on the Thai Embassy London website list to issue a COE?

I think something might have been lost in the translation here. The only related item I can think of is that those coming to a sandbox, in addition to being vaccinated, cannot be from a red list country. That probably means that you cannot pop from a red list country to a lower risk country and immediately get a COE to travel into a sandbox.

I could not find it on the London embassy website. (infographic https://image.mfa.go.th/mfa/0/umufy3EgqL/Phuket_Sandbox/Revised_PR_Eng_Phuket_Sandbox.png) 

I did find it on on the embassy website in the US. 

Source: https://thaiembdc.org/phuketsandbox/

 

Edit: Further checking on the COE application website I found it on a popup (number 2 under 1) you get when completing the application and selecting the phuket sandbox.

Link for image: https://coethailand.mfa.go.th/images/Phuket-Sandbox-en.webp

 

 

 

What’s also open to interoperation by the officer on the day I guess, is if you must stay in one country that is acceptable for 21 days prior or can you have done a week in 3 different acceptable countries for a total of 21 days? 

I just got my COE for the Phuket Scambox approved Monday this week from the New York City Thai Consulate.  On submitting my application (On what would have been a late Saturday night in Thailand time), I received a response within an hour via email asking for a signed letter guaranteeing that I had stayed in the USA for at least 21 days.  I had no idea this existed and thank goodness I had purchased flights that gave us literally exactly 21 full days in the US.

 

Anyways, wish this had been made known clearly before, I guess shame on me for missing something in the fine print, though I researched all this very carefully before departing.   Thank goodness we just happened to meet the 21 day minimum.