Very bad behaviour on the part of Boris, but the real concern should be the timing of this leak - and the why.
Just as Hancock was got rid of to let someone more amenable to letting 100s a week be killed by Covid (and many more disabled).
Once Boris is gone, we'll be told up to 300 deaths a day is a stable 'low' (perhaps we'll also see a rise in Covid caused heart attack, blood clot, stroke, and sepsis deaths).
Case numbers are already not counted due to test shortages falling.
Now is Boris's chance to emulate his idol, Churchill, who stood almost alone in the Commons against support for appeasement re WW2. Like his idol did in WW2, Boris should work with the opposition for the good of the country - and it's people. Go out with head held up high, sticking two fingers up to his backbenchers.
Yes we need to keep the economy going as much as possible but we also need to 'live with it'. Living with it a disease that kills and disables means basic infection control mitigations. Masks, vaccine passes, and good ventilation (eg. HEPA filters and corsi-rosenthal boxes). Further down the line we'll have more of the rest of the world vaccinated, better knowledge of a still new potentially unnatural virus, and wider availability of the currently limited supply of treatments. Ideally we'd get there without needlessly letting the bodies pile up or get disabled.