Before the usual suspects get carried away with the popular passtime of kick-the-MoD, there's a whole raft of reasons why the Equipment Budget has underspent. Without sight of the individual elements, though, we don't know what they are likely to be. As Bastardeux suggested, it could be tactical but that's a dangerous plan where those treacherous sods in the Treasury are concerned.
For all of you who are worked up about cuts to the Manpower Budget, unless someone's changed the rules, you can trade bodies for kit but not the other way round.