Without meaning this as a comment towards your post in particular, I've found that people who say "they're voting against their own interests" tend to project their own solidly middle-class interests (access to abundant, cheap labour, better deals on fees to go to university, etc - completely valid self-interests to have!) onto working-class people who may be unaffected or even harmed by those interests (competing with that same labour pool for work, being outbid for housing from the rest of the population, being unable to afford to have a child).
What this also misses is that people have self-interests besides straightforward economic metrics. Is it not a valid self interest for someone to want their home town to maintain a sense of local identity? For a community to not change beyond recognisability in a single lifetime? For a person to not feel like a foreigner in the only place they've ever been able to call home?
These concerns aren't as strongly felt by the more well-to-do people, since they have the means to live a more cosmopolitan, travelled lifestyle, and give their lives "higher" aspirations and meaning, like academia, or activism. But for poorer people, their home, and local identity and sense of community, is all they have. Like you said, there's little sense of collectivism in the UK, and is it any wonder why when whole communities have been either displaced by market forces, or replaced demographically, in less than a single generation?