The executive, which in effect means the government has too much power. A republic may limit this, as an elected President may claim as much legitimacy in actively playing a role in the system. As things stand, the strongly centralised Labour government can stick two fingers up at Parliament and do what they like. Parliament can't act as a check on a strong, disiplined government with a majority of over 100, let alone 164. And PM's question time is a joke. All questions have to be submitted in advance, and the conduct of the MP's is more about petty point scoring than getting to the bottom of issues. 30 minutes a week is far too short a time to challenge a PM about the conduct of his/her government.
Agreed Why do we need a President? We already have our own Prime Minister (Jean Chretien). The Queen is just a figure head and as far as I know doesn't hold any real political power. I don't really want a President.