Where to begin, sadly...
The Tier system (especially up to Tier 3) has obviously been inadequate in dealing with the threat, given the need to introduce a fourth tier (and, more than likely, a fifth tier in a few more weeks). So there's that for starters. More generally, it's just that in the last few months, Government policy has been reactive rather than proactive. Take the announcement about delaying school reopening. That should have been made some weeks ago, and the Government at the very least should never have threatened legal action against schools who tried to pre-empt the threat by closing early.
From October 15th to December 25th, the UK has announced around 27,000 Covid-related deaths. At the start of that period, I suggested that limiting that total to 5,000 (later revised to 10,000) deaths in the same period would represent a success. Perhaps I was being wildly optimistic, and that already the die was cast, but then again this is my exact point: in order to keep excess deaths low, the Government would have needed to take action far earlier than it did. Introducing Tier-3/4 measures when the disease is already out of control is far too late.
That's what I mean about the Government not taking the threat seriously enough: it keeps waiting far too long to take the action needed to halt the spread. Thousands of people have paid for that inaction with their lives.