It's a pity for Blair that his Presidential leadership is entirely unofficial. Were he a President, he could switch parties to the Conservatives. He would win a loyal party, and the Tories would gain a leader with some modicum of charisma (a bit of a 'wet', but not really all that left-wing). But since he's the Prime Minister elected by Parliament, his career looks fucked. It's not unlikely that he would lose a leadership challenge within Labour (unless he did a deal with the Tories to establish a "government of national unity", giving Tory frontbenchers cabinet positions in return for their vote of confidence. Which could yet happen.)
Though it's sobering to think that the only opposition to Blair's Labour party is the even more hawkish Tories. (What is the state of third parties in Britain? Where do the Liberal Democrats stand, and how much clout do they have? Is there anything like the Australian Greens there?)
The Lib Dems have some good policies and usually poll around 15-20 percent, so are in a stronger position than the Dems in Oz, but have no central government experience and are really seen as the party of disaffected voters from both sides. You can never rule them out, but structural change within Labour is probably more likely at this stage.
The Lib Dems are currently anti-war, certainly anti any war which isn't through the UN route. A lot of their support is from the middle-classes-with-a-social-conscience kind, as well as cast-offs from the other two - Tories who are a bit wet, or Labourites who are unhappy with Blair's presidential style. The Lib Dems were also quite prominent on the anti-war demo the other weekend.
There is a Green Party, but it isn't close to winning seats. The only significant parties to the left of Labour at the Scottish Nationalist and Plaid Cymru (Welsh nationalist) parties - significant in Scotland and Wales, but not in the UK as a whole.
The strongest political opposition to war at the moment comes from the Labour left and other disaffected Labourites (a number of former ministers have spoken out against war), Lib Dems, a few Tory wets, and the minor nationalist parties, in that order...
Tony Blair is exactly the leader the stricken Conservative Party needs, and you're not the first to <a href="http://politics.guardian.co.uk/Print/0,3858,4606424,00.html">suggest a swap</a>. But the Government's power within the legislature is so great that Blair doesn't need to make deals with anyone. Labour Party MPs remember the disastrous consequences for their political careers of their last attempt to split, in the early 1980s.
The UK's voting system has a structural bias towards large parties (at least in Parliament; the tentative introduction of regional government has seen some very different situations). The Liberal Democrats win a consistent protest vote of around 20% but have only 8% of MPs. This allows them to endorse alternative policies without serious commitment, and their leader has been a vocal critic of the war. (The Economist believes he'll prudently keep quiet when British troops start fighting.)