I know HoL but, as I understand it, 1, a government can pack the HoL with its supporters and 2, convention dictates that if its in the manifesto, the Lords can't block it.
The courts could also prevent it (?) But if we exit the ECHR, as many want, well that's that option to stop it in the bin. Don't know about Supreme Court.
The King, I believe he just does as he's told by government so doesn't have much power. Also, I believe (?) government can sack him if they want.
Was talking about this at dinner and couldn't think of anything, that, in the right circumstances and with popular public support could prevent this. But I don't know much about anything 