That isn't really the case.
The Monarch's useable power has diminished over the years, but it was well after Magna Carta - and the reserve powers that still exist today and could be used in an emergency are still very real. The actual Monarch hasn't used their reserve powers in a long time, but in my country (Australia) the Queen's representative, the Governor-General last used them as recently as 1975 - he sacked the entire government to resolve a constitutional crisis.
More significantly, ultimate power to command the military rests with the Queen. The emergency orders that would release nuclear weapons in the event of an attack of the United Kingdom are 'Queens Orders'. Yes, the Prime Minister has the power to issue them, but they are the Queen's orders.
And if the Prime Minister is killed or incapacitated, there is no automatic succession. A new Prime Minister must be commissioned by the Monarch. However, if the Monarch is killed, the next person in the line of succession instantly becomes King or Queen. For this reason, maintaining the Monarchy in the event of an emergency is more critical to British survival than maintaining the Prime Minister. The King or Queen (whoever that may be) can govern alone if necessary. A Prime Minister cannot.
Thanks for clarifying that for me. I knew that the monarchy in England didn't cease to rule immediately after the Magna Carta was enacted, but I knew that somewhere in there the majority of their power and authority was gradually handed over to the Parliament and the bulk of their duties became more ceremonial. I just wasn't clear on the division of powers.
Thanks to your post, I understand it a little better.
Thank you for explaining that so well showing how important the constitutional monarchy is and in real terms the position of the PM is relatively unimportant after all it is the party that is voted into power by voting for your individual MP not one man and the government is run by cabinet collective rule not that of one man.