The point is though that it is her duty to ensure the continuation of gvt. If there was a stalemate and an impasse over who was to be PM then it is the monarch who has final say. It is the duty of Parliament to never put her in that position. However who'd have thought a President of the US would try and override their system? Peter Hennessy lays all this out pretty comprehensively in "The Prime Minister, The Office And Its Holders Since 1945"