Key Point Defence
The MoD has long maintained a list of 'key points'. These Key Points are sites where security is essential to the "... ability of the country and the armed forces to conduct military operations."
Apparently there are currently around 160 MoD key points. They include such sites as "... the BT Tower, the Foreign Office, 10 Downing Street." Now to be honest I don't see Gordon Brown wanting to be reliant on a bunch of rough squaddies (who might remember how he shafted them on equipment/pay/accomodation both at home and in Iraq and Afghanistan) for his personal safety. A ND might just happen when everyone was least expecting it!
There is also a separate list of economic key points whose protection is the responsibility of the Security Services, as part of the protection of the critical national infrastructure. However as the Security Services are mostly profitably deployed hunting down transgressors of the Road Traffic Act, non-payers of the Television License and arresting people for being aggressively white in a muslim area, there's going to be a major shortfall in this area.
Now exactly how a much weakened and mostly deployed overseas UK Armed Forces is going to protect every letter box, ATM machine and Labour Party members gaff has not been explained to this call sign ... but one looks forward to seeing what the plan is when the balloon does go up ... and its probably going to be quite soon.