It's also the middle of winter. You know, when it's easier for viruses to spread and people's immune systems are generally weaker...
If you're in a Tier 3/4 area, you can't 'go out' anyway. You can go on walks, to the supermarkets/shops and to the gym (and with how strict they are on rules there's no evidence that they're hotspots for it), that's about it.
It was always going to get worse at this time of year.
Also, it looks like it's mainly London and the south east where the hospitals are filling up - the ICU ward at Sheffield Northern isn't full, and neither is Leeds (as of yesterday, they had 12 covid patients).
Hopefully it stays that way, but the risk of all of this would have been minimised/lowered had London and the SE been placed into stricter lockdown at the start of this month.