oak so all you can actually do is advise people to do the right thing, and then shield the vulnerable and the elderly to save as many lives as possible. There is no point closing down with a lockdown if the compliance is as poor as you describe.
If we are not prepared to order a police state (there would be zero support for this in the UK) then the only other option is to put some measures in place to 'slow' the spread as much as possible, and hope to God the hospitals hold out in the winter.
We let young people carry on with their lives, we continue to keep the economy alive and well, we protect those at most risk. What else can be done? We have to decide and quickly. The straddling of both options does not work, as we have seen and will become increasingly more obvious as the infection spreads.