I guess "people changed their behaviour anyway, so we might as well have had restrictions", I guess breaks down on who and how those behaviour changes hit.
Isolation and similar behaviour change does have significant health costs itself but like any it's not spread evenly between individuals. So if the messaging was wrong and people vulnerable to isolation isolated themselves then that's bad. But if it was only people who are less vulnerable to isolation then it doesn't matter so much.
It's been the same utterly failed communication from Whitty and public health england (and specifically them because that's their remit, specifically about public health so everything needs covering and not just covid harm) that lockdown and isolation is immune from harm, rather than a balance of harms.