Another major expendature, which is significantly higher than our chinese, russian and indian friends is the wage bill. We earn considerably more than them and each of us has about 6 civil servants each to make sure the cogs of defence turn correctly.
Even taking wild stab in the dark figures, if there are 200000 people in the forces and each of them earned 25k, that would be 5 billion straight off the budget. Then add on the pensions to ex forces, equipment costs (new and maintenance of old), Defence estates and of course plasma telly's, its starts to become obvious that defence is an expensive business. Its all very well having a huge army like china, but we could probably afford a few more soldiers if we made them live in poor conditions, gave them obsolete and poor equipment to use and paid them next to nothing.
Its all swings and roundabouts as they say.