I think you should vote for someone you believe in, rather than voting for someone who is not someone else

This would be the ideal situation, but for so long as we have first-past-the-post it's a fundamentally ineffective way to vote. Thanks to Duverger's law, unless one of the two big parties just so happens to coincide with your views then the best you can do is to vote against whichever of the big two you dislike most. "Big two" here depends on your constituency - it may not be Labour and the Conservatives locally, but it is true that virtually every constituency has at most two realistic options. Labour may not be very good, but if they're in power it's probably at minimum going to make this a better place for asylum seekers and trans people (or whoever the Tories would go after next), and Labour's voting record on the environment really is far better than the Conservatives' too.