I would say there is some reason to vote in the UK because even if shitty politicians are guaranteed to win, not having an overwhelming majority of the vote still means the parties need to at least put up a façade/not go all in on shitty beliefs, because there's still the threat that people won't vote for them if they go too far.
But the UK seems to have even less of a dichotomy between the 2 major parties than the US, so it's pretty fucked... the labour party is supposed to be the "leftist"/populist party, but they're pretty much indistinguishable from the tories most of the time if not all of the time it feels like. So I completely get why the British would feel completely and utterly hopeless even compared to Americans