The problem, as others have pointed out, with a 'full lockdown' is that it isn't full. It can't be. Supermarkets, hospitals, carers for the elderly or disabled plus other health care people like district nurses, transport, water/gas/electric all have to run - imagine winter if the gas or electricity workers are shut down? Supermarkets have to be supplied, so farmers, abattoirs, warehouses, factories all have to keep running. Those workers have to get to work - so buses and trains have to keep going. And you have to keep the garages open so those who use cars can get petrol or parts for the car. Manufacturers of bus/train parts have to stay open or you quickly get buses off the road for the want of a fan belt for example.
There are a huge number of people who are 'essential' that we don't actually see - but they supply those we do see. And they will come back to their families, potentially with Covid. Plus, of course, those workers won't be able to work if their kids aren't at school. If you have a primary school age child who do you leave them with if you work in a factory and your DP drives a bus or is a doctor? Because in lockdown that child care will not exist - it didn't last time.
Gyms closing down make some sense as they are confined spaces where people mix. Those people are usually breathing hard and sweating so the viral output - if they have it - would probably be more wide spread. Some have put in excellent measures for containing viral spread but this virus is so new we really don't know if all this cleaning etc works.
And on that note, when was the last time your place of work did a swab test on surfaces to see if the chemicals you use actually kill Covid. That's assuming such a test exists?
Pubs and hospitality is a slightly different thing. Most pubs and restaurants are pretty Covid clued up. But it's the loosening of common sense that goes with having a few beers/glasses of wine that's the problem. It's when people leave that the mixing starts. Quite how a 10pm curfew helps I don't really know but the issue is getting people from the pub to their home without them jumping on each other, fighting, giving 'you're my best mate' hugs or inviting half the pub back to their flat to continue the party. Solve that and the pubs could stay open.
As for shielding the vulnerable. We have a DS who has a heart condition. He is allowed to go to school according to his consultant. If he weren't, DH and I would have to shield as he's 13 years old - he can't look after himself so can't shield alone. And what about those elderly people with dementia living at home? Those with disabilities or illnesses that cause mobility problems etc? If they can't have carers (some of whom may live with them - family or a live in carer) then they have to be found a hospital or care home bed because they can't take care of themselves. Shutting everyone in their homes is much harder than it sounds. Humans mix, they are communities of very diverse groups of people. And because they're not all young, fit, able to work from home etc you can't stick them into silos and hope for the best.