No left leaning party can do what UKIP did because UKIP threatened Labour as well as Tory seats. They were a single issue party and could run as MEPs which gave them another soapbox to shout from. There just isn't an issue that has the same kind of cross-partisan reach yet.

I think abolition of the monarchy will be that issue in 10-15 years time. Once the older Christian conservative voters die off there will be a stronger push to move to a republic. It's something that a single issue party could run on and steal votes from the dominant parties.