"Why not impose a lockdown on the unvaccinated?"
We don't have the infrastructure to police that and it would need to be policed
Correct
Shielding was never compulsory or policed
Correct
so it cannot really be said that if one can be done so can the other
Yes it can, if what you are saying it that these people need to be removed from ordinary circulation because of the threat they pose to the whole. This evening told to shield overwhelmingly did so without need for compulsion (for there was a self interest there too) and society was OK with millions of people being in a form of lockdown. A different form of lockdown would be needed for a group where there was less expectation of voluntary compliance.
But when there are so many reports that 80% of those needing ICU are unvaccinated, and the remaining 20% are those who are highly immune suppressed, then taking this effort people put of general circulation would have a disproportionate effect on NHS and therefore for society in general.
And if it's OK to shut away the shielded, then it's time to do similar for the much larger group as well.
Remember as well - shielding had no age categories, and the people asked to shield weren't generally moribund or dependant (though some were), so the economic effect of taking them out isn't unusually low or insignificant
I don't think what is being asked of the shielded is really ever considered when people trot out the 'isolate them' ideas. If it's wrong to isolate people who have chosen not to take steps to make themselves less likely to fill up ICUs, then why is OK to condemn those who can't help it to ongoing isolation and all its consequences?