Gordomac
First off, I believe Jet2 are to be commended in taking the lead; you might have expected that perhaps TUI would have been in the vanguard of delaying once again the summer start. I was just scanning through the new case 7 day incidence from Johns Hopkins University and frankly you just can't see the UK turning any major holiday destinations to "green" in May, except possibly Portugal who's incidence has fallen dramatically, and Malta. Spain, Greece, Cyprus, Bulgaria and Turkey are not going to be anywhere near by mid May, so the hope must be thing may have improved from both new case incidence and vaccination rate perspectives. Jet2 have made a good call, even though it won't be universally popular.
As regards your other point regarding vaccine certification, I must admit I was under the impression that AZ, Moderna and Pfizer had all been fully certified (by UK). If they haven't and travel insurance in thus voided then that's a rather large can of worms that could be opened.