I want to be clear here that this is dangerous messaging. While any individual vote likely has little effect on the outcome of an election, it's people's collective vote that does ultimately decide the outcome. And when the electorate is disengaged, disinterested, and apathetic, that is the environment in which fascism and authoritarianism thrives. Voting is not and should not be the end of a citizen's political participation, but it is still vitally important. Voting should only be the foundation of citizen political participation. It's also important to campaign, to discuss important political issues with others, and to protest and take direct action against the injustice of the political class. But if you don't vote and spread the idea that voting is meaningless, your efforts will change nothing.
This line in particular comes a lot of young people, and it is an absolutely understandable and reasonable conclusion for them to come from seeing as they are the most politically neglected group, and politicians almost never pay more than lip service to the concerns of the young. Youth turnout in elections is historically rubbish, so why would any rational politician pay heed to the demands of a voting bloc that won't influence the outcome of an election? Politicians who pander to youth voters will lose to politicians who pander to old voters simply because youth voters will stay home while old voters will show up at the polls and vote their guy into office.
It costs almost nothing to vote and to encourage others to vote as well. So do it. It is irresponsible to spread the idea that voting is meaningless without also attaching the context that if you don't vote, you have no power at all.
The UK is a small country that doesn't have nearly the geopolitical sway that the US does. If the UK withdraws to itself, that sucks for Britons but the rest of the world will carry on. If the US withdraws to itself, it will suck for the rest of the world but the Americans will carry on.