Basically, in Britain, if one party is elected with a huge majority it can lead to elective dictatorship. This is one of the phenomena of FPTP and one of the major criticisms of our electoral system. Before Blair's backbenchers started turning on him for instance he could force through most legislation despite opposition just because he had a mojority of around 160 seats. Power is also further abused because the UK has an uncodified constitution as opposed to the US Bill of Rights etc - a codified document. Our constitution has evolved alongside the state and is therefore quite flexible, but it does mean that the power of the PM is not restricted as much as would be desirable.

In Britain the monarchy doesn't really come into it - the head of state - the Queen - is merely a figurehead who signs legislation.