We're way past lockdown. Had we wanted to go down that route we should have done it back in January. That being said, how long do you then go into lockdown for? Would the public stand for it?
Going into lockdown now would simply be another delay tactic, as the virus is already rife throughout the country. Yes, official number do not back that up, but you have to assume this given the high proportion of 'well-known' people who have tested positive. Our peak would stay flatter but we'd be open to further waves with no significant immunity.
You're right that herd immunity is the way the government are going. Scarily that will involve 70% of the population getting infected if it were to be successful. Delaying this strategy isn't ideal. You're right that the virus is prone to mutation, though this doesn't affect our herd immunity plan. Examples from Spanish flu confirm this in Copenhagen, where the deadly second wave barely caused a ripple due to widespread immunity from the first wave. Interestingly, Denmark are not going down this route for Covid-19.
I have faith in the government, despite not being a Tory, that this is probably the best way forward, at least now because we're not in a position to implement other potentially effective measures due to it being too late to. I agree that a better lead from them is needed. I hope that our course of action, a unique one at that, proves to be the best one longer term, though I still have some doubts (it is Boris after all). Who knows whether a second or third wave will develop, and how deadly they'll be?