Brits are pretty much a two party system, though not for the same reasons we are. So if a UKIP rises, it will likely come out of the conservatives membership. They would still be a two-party system, unlike France or Germany in 1928-32, or many other Euro countries, where (and I keep saying this) parties LIKE the 1932 Nazis or the 1980s Green in Germany can get power they otherwise would never get.
The two-party system is a miracle of stability. It’s a fragmented parliament that leads to the Nazi Party or it’s modern equivalent the Greens gaining control. They’re minority parties that don’t deserve power ever.