In the 2011 Fiscal Year the UK Central Government spent £512.2 billion. Only £35.4 billion of that went to the military. The major budget items are pensions, heath care and welfare. The is what drives the public debt.
While that is true, it’s only half the picture. Once the budget cuts begin to show through in the current year figures, you will see that Defence has been cut less than most other spending departments. It’s losing only 8%, whereas many are losing 25% and some as much as 40%. It certainly isn’t the case that Defence is losing out in order to keep benefits spending intact.