"people in this country are too selfish/stupid/libertarian to prevent this from happening"
Certainly an element of that. House parties, risk takers, virus deniers and DILLIGAFs. Also other factors: the working poor; those who have to use public transport; some who were probably just unlucky.
Three relatives have had Covid that I know of. One lives in London, the epidemiologist. He was infected during the first wave. The other two are on my partner's side of the family, they don't live in the UK. They're from a country which had hardly any cases in the spring, maybe people got complacent as they've had a bad second wave. A cousin caught the virus while on an ill-advised beach holiday; my brother-in-law caught it fairly recently, most likely at work.
You're more at risk the more people you have contact with. Avoid those kinds of situations as much as possible and you're likely a bit safer.