Although Blair (and many other politicians) forget the fact, the PM owes his/her office to the Sovereign, who invites him/her to form a government, and who may dismiss him (the understanding that this would be unconstitutional except in extreme circumstances notwithstanding, the right to do so remains).
In 1975 the Governor-General of Australia, who is the Queen's representative and appointed by her, dismissed the Prime Minister Gough Whitlam but I don't think she can do the same in the UK. As I understand it she certainly can't dissolve Parliament. I think Mrs Thatcher is the only Prime Minster removed from office without an election.
Read more at
http://en.wikipedia.org/wiki/1975_Au...utional_crisis