it's a way of circumventing the quarantine restrictions for the UK. Direct from Thailand means quarantine, fly into a country not on the red list first for a few days and then go to the UK, means no quarantine
If you are departing from Greece, you get the COE from the Greek Embassy. I flew in from New York 2 weeks ago and used the New York Consulate (British citizen) for the COE.
I flew into New York a few weeks ago (Back in Thailand now) and there was no quarantine at all. I flew with Etihad and they required and checked your covid test (must be taken 72 hours before flying) during check in and during the layover. I think US Customs and Immigration checked as well.
Asides from that, everything else was like flying normally