The problem is that if elections are won from the centre (as I think you've said before) then if centrists continue down the well-worn path, voting for a no-hope candidate won't make a difference. And your UKIP analogy is only partly valid, because UKIP voters are, by definition, not from the centre.
The best hope for change is that Green politics gain enough traction due to environmental concerns, that they become the 'new UKIP' in terms of wielding influence disproportionate to their vote share. I've started voting Green in local elections for the last 5 years or so. Made no difference whatsoever, the candidate usually polls in very low triple figures and it barely moves from one election to the next. I've voted LibDem in general elections for decades, for the same reason you recommend - I'm not voting for 'least worst' but for change. Again, it makes sod all difference. I have to ask myself: if I'd voted 'least worst' rather than 'neither of those, thanks' would we be in a better place now?