I think that is the difficulty - utilitarianism is an enlightenment era philosophy - and a product of its own times and agenda and a reaction against early modern mysticism and religiosity. An attempt to impose rationality and quantifiability on the vicissitudes of human life and emotions.
But is a very blunt quantitively instrument - based on a false premise that we can somehow know, quantify and predict "happiness" and make judgments accordingly. But these don't necessarily have much to do with reality.
First of all, it presumes that "happiness" should be the goal of life. But there are other schools of thought which priorities other ideas - such as, thinking off the top of my head, honour, worship, service, productivity and all sorts of other things. I am not saying I agree - but there are other ways to order society.
Second "happiness" is notoriously difficult to quantify. What is happiness? Are you happy? Are you happier than your next door neighbour?
The reality is human experience is not linear, predictable, general; unfortunately it is complex, messy, and contradictory. We can't in reality make judgments based on "greatest happiness to the greatest number" without falling into the trap of omnipotence, thinking that we can somehow "decide" and "calculate". It is a false soothing to our consciences - a false justification not based on reality and pseudo-scientific.
It seems to me that the only thing to do is to treat the opportunity to continue living as morally the most important thing. If that means some people have to endure a hard time in order that others can have the opportunity to live, then that is how it has to be.
I am not saying it is fair (it is not - some people suffer much more than others under lockdown) nor easy (it is not!) but I just can not see that, (at least in a first world country with welfare system, free healthcare and free education) that my economic position/my children's schooling or even my mental health ... should take precedence over someone else's actual LIFE. I will have my life at the end of all this and still the opportunity to do something with it and make something of it, and to recover from the difficulties I have had to manage.
If we didn't have lockdown many more people would have not had that opportunity - as it is 100,000 have died and that figure would be much greater with no lockdown.