I think the issue is that cabin staff cannot enforce what is government regulations other than the ANO “not complying with a lawful command.” as it is the job of the police under the The Health Protection (Coronavirus, Wearing of Face Coverings on Public Transport) (England) Regulations 2020
My concern is that the Greek islands are the summer equivalent of Ischgl, people in small local accommodation coming together for a good time in the evenings in the bars that are open until all hours.
It’s a possibility that that the passengers caught the virus in Zante rather than the aircraft, in which case it’s time the government started checking out all passengers that have recently returned from the Greek islands otherwise there could be a rapid increase in numbers. The problem is that 80% of carriers show no symptoms and will only be identified by a Covid test.