% of GDP is the best way to look at things as you get an accurate look at how spending changes relative to income. All figures quoted are the total of central government and local government spending. Here's a look at what has happened since 1997:
1997 2001 2005 2008
Pensions 6.060% 6.807% 6.998% 7.308%
Health Care 4.976% 5.408% 6.721% 7.515%
Education 4.553% 4.522% 5.126% 5.615%
Defence 3.091% 2.878% 2.721% 2.819%
Welfare 7.895% 6.173% 6.366% 6.610%
Law&Order 1.891% 1.880% 2.294% 2.372%
Transport 1.225% 0.908% 1.294% 1.448%
Gen govt 0.796% 0.952% 1.587% 1.869%
other spending 4.611% 3.999% 5.065% 5.492%
Interest 3.444% 2.624% 1.971% 2.248%
Balance 0.476% 0.426% -0.268% -0.388%
Total 39.019% 36.578% 39.874% 42.908%
Other spending consists of agriculture, forestry and fishing, waste management, government sponsored R&D, broadcasting, pollution abatement etc.
You can see certain trends over time. Health spending is jacked up (though our spend remains lower than that of any G7 country with the exception of Japan). Spending on state pensions goes up due to increased number of older people, plus Labour increasing the state pension through minimum income guarantee, plus other payments and free services for the elderly (free eye tests, free local bus pases etc). Spending on education and law and order is also up.
Spending on welfare goes down due to unemployment dropping. In future, as the reforms to incapacity benefit kick in, it should continue to drop. Spending on debt interest drops too - Labour spent the first six years paying off debt, and renegotiated a bunch of it at lower interest rates and most debt issued by this govt has been at low coupons (the lowest being 4% in 2005).
Many Tories believe that a potential conservative government would cut tax - but Cameron has boxed them in somewhat by his statements. He has said that the NHS is sacred and they wouldn't make any cuts there (and given our low spend relative to other countries, pretty hard to shave spending in this area without trying to destroy the service and returning it to the shambles of the Major years). They would probably not touch state pensions as most of their supporters come from the over 60's. The over 60's are the stroppiest part of the electorate - despite Labour spending a lot more on their state pensions, they still believe that they arn't getting enough. Tories have signed up to Labour's education policy. They want to increase defence spending, and it's doubtful they could do better on welfare spending than Labour (Labour's record on reducing welfare spending is much better than the Conservative one).
Which leaves them with cutting back on such vital things as waste management and pollution abatement etc. The Thatcher and Major governments paid for their tax cuts by simply privatising assets. Essentially previous Labour governments used to go to the pain of raising tax in order to buy out shareholders and nationalise assets. Then Tories would take this store of money helpfully put away by Labour, and sell the assets and spend the loot. New Labour hasn't played the nationalisation game though. We've nationalised just two entities - Railtrack (which no one wants to buy) and Northern Rock, which people might want to buy once it's back on it's feet. Northern Rock's market capitalisation was about £4bn prior to it crashing - raising that in a privatisation is peanuts in the context of government expenditure.
So I'm very interested to hear what the Conservatives have to say at their party conference. The only way they can supply tax cuts is for Cameron to abandon his commitments to the NHS, state pensions and education. But of course those are the very commitments that his populatity is based on.