The PM is simply the leader of the party which has the greatest no. of seats in Parliament.
If Blair got kicked out, but Labour still won the election, then presumably Prescott (as Deputy PM & Deputy Leader) would be PM... at least temporarily.
EDIT:
D'oh. Forgot to mention about what MB said. Have to be an MP too.
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Reg Keys, father of a soldier killed in Iraq, is standing against Blair -
http://www.guardian.co.uk/guardianpo...466080,00.html