Everything is eventually decided by the majority of votes in the house of commons. Even if you put a law in saying that the pm can't do this without a 80% vote, that law itself could be repealed with a 50% vote.
Theoretically it would only require a 50% vote to remove elections or something crazy. (Although in practice that might not get past the king who technically has the final say)
There is no formal constitution that has more protection like in some countries.