As I understand things, they are not "viable" but certainly will cause the Party In Power to either favorably consider a forced coalition or see some of the working of government delayed if not outright halted.
I say that based on the presumption that "viable" means they can stand on their own two hind legs rather than needing to ally with another party. That's the historical "It's Labour's turn to be the government/It's the Tories turn to be the government" exercise.
That being said, I see an opportunity for the Loyal Opposition to be joined by an Unloyal Opposition, forcing an actual discussion that has better odds of leading to actual change(s).
stay safe.
