I also commend this analysis from the Institute for Fiscal Studies:
www.ifs.org.uk/uploads/gb/gb2015/ch9_gb2015.pdf
It points out that total social security spending is forecast to be £220 billion in 2015/16, around 30% of total government expenditure. I quote directly below.
Pensioners are expected to receive 55% of social security spending in 2015/16. This proportion has been growing as a result of increased numbers of pensioners, greater state pension entitlements among those who recently reached the state pension age, and the fact that pensioners have so far been largely protected from the cuts to social security. Continuing to protect pensioners would require much larger cuts to working-age social security for a given reduction in public spending.
The Conservatives have said that they would seek to make further cuts of £12 billion to annual social security spending were they to form the next government. To give an idea of scale, freezing all benefits and tax credits other than state pensions for five years would cut spending by £13 billion, taking an average of £800 a year from 16 million families. To cut spending on this scale while protecting pensioners entirely would require more severe cuts to working-age benefits; even continuing the Conservatives’ proposed freeze of most working-age benefits for five years would only reduce spending by £6.9 billion.
Other options that could save substantial sums include making all tenants pay at least 10% of their rent (£2.5 billion), abolishing child benefit and increasing universal credit to compensate low-income families (£4.8 billion), reducing the generosity of means-tested support for children to its 2003/04 level (£5.1 billion) and restricting benefits for families with children to the first two children (which would save around £4 billion a year in the long run).
Many of the policies suggested by the Conservative and Labour parties - withdrawing winter fuel payments from higher- and additional-rate taxpayers, cutting housing benefit for young people, reducing the benefit cap, and increasing child benefit by 1% for a further year --- would reduce spending by relatively little.
The social security system not only gives support to vulnerable groups but also affects incentives around how much paid work to do, where to live and with whom, and even the number of children to have. Giving exemptions from cuts for groups deemed more vulnerable can weaken work incentives and strengthen incentives for people to have children or claim disability benefits. When considering possible changes to the social security system in the coming years, policymakers should bear these trade-offs in mind, have a clear vision for what they want the social security system to achieve and ensure that the overall system of support is coherent.