@Svenk
There are 4 million UKIP voters but they are too widely dispersed to win anything under the FPTP system.
If you want politicsl clout, you will need to do what minorities do: all move house to live in one borough, so as to get one of your own into parliament.
There are 3 million of them but mere handfuls of minoritu MPs and, even so, they follow existing political flags - because unafilliated independants so rarely win seats and no-one can be bothered to track their contribution when they do.
The Greens are, I admit, an anomaly, in terms of coverage versus popularity. But there's the rub: no coverage makes it impossible to glean new voters. Most of us are resistant to single-issue indrpendants & parties because they (seemingly) don't address our hundreds of other daily concerns.
As we know, the Greens have moved on from single-issue but, if squeezed hard enough by interviewers, what emerges is an underlying deep socialist streak, which might make Corbyn blanch. (Or perhaps they should merge with Corbynite Labour?)