In the past, police power was often used against the mob. There were many occasions when the state regarded the citizens as the enemy. This is more clear in continental jurisprudence than it is in our British tradition. The ultimate law is the safety of the state. That's an old legal principle—lex ultimus salus populi est.
The state has always had the power, whether it admits it or not, or whether political theory admits it or not, to ensure that the state itself survives. In that sense, an attack on a minister of Her Majesty's government is a threat that I think the state, in the form of Parliament, has to respond to. Surely a basic element of our democracy is the right of elected members to go about their business.