Stick a few silos on Diego Garcia for the end of the world and to placate our "we are still a big power" side, and then please start spending the money on defending the coastline thats 10 miles away and the airspace above my head.
Lots of bigger, wealthier and equally important countries as ours seem to manage! £25 Billion buys a lot of stuff, not that it would go back to the MOD.