Lucca In my opinion (for what it's worth), it's because the UK was never tough and consistent enough with lockdowns that we're in the state we are. Add to that the fact that Test, Trace and Isolate never worked effectively, despite having billions thrown at it.
The latest fad seems to be mass testing, which is incredibly expensive and won't achieve much either, unless people are forced to self-isolate (with necessary support). In schools, even those who have been in contact with an infected person won't be sent home. Instead, they will stay at school and be tested regularly (well, that's the half-baked plan).