It distrubs me that I was about to come on here and ask this exact same question.
what annoys me about this is that the arguments saying that the Prime Minister has become more presidential are just so weak. If you compare prime ministerial government to parliamentary and then see how the UK system measures up they are still totally different..
But my answer essentially is:
Presidential system: President elected separately to legislature, fixed term, head of state, tends to be based more on personality then party, president can choose who he likes to be in the executive, President manages budget, can appoint supreme court judges, essentially head of foreign service,
Prime Ministerial system: Executive is accountable to and drawn from Parliament, not head of state, tends to be more party based, Prime Minister must choose his executive from members of Parliament.
PM increasingly like head of state
Increasingly control of foreign office (i.e. Blair)
Personality over party (Blair versus various Conservative leaders)
Special Advisers/Lords means PM can appoint not just MPs
PM has large input into the budget - Blair particularly, Brown will probably be the same
With 2008 UK Supreme Court PMs will be able to directly appoint the judges of highest court of appeal