//and they think it is democratic???? //
the leader of any party is a matter for the party. bear in mind we, the electorate, don't vote for a prime minister at a General Election, rather we vote for an MP to represent our interests. once all the MPs have been elected, under our party political system, the winning party is the one which commands the biggest majority and is more likely to be in a position to govern (ie have enough votes in parliament to schedule taxes, etc). the leader of that party is the one invited by the head of state to form a government, and thus becomes prime minister. if the PM stands down as party leader, it's for the party to elect a replacement. once a replacement has been selected, assuming that party still commands the biggest majority, the new leader is invited to form a government. the new PM may choose to call a G/E (as the last one did) to obtain their own mandate, but there's no requirement to do that - neither James Callaghan or Gordon Brown did so when they became party leader.