They're probably not "Beat" cops, there are highly paid special branch and detectives too. Lets say on £80k each. There's all the intelligence activities that go into this. As somebody else mentioned overtime, other costs such as surveillance equipment, maintaining a situation room, heating, and food, helicopters, public relations christ the costs go on and on ...
Heh, Yeah I didnt really mean "beat" officers, per se... but I am refering to officers that are simply guarding an already guarded building for political reasons...
I hadn't thought of the officers as being some elite force -- though I have no idea if they are, although assuming they may be lowly "beat"-like types is not a good assumption. :-)
Hahahahaha - "beat" officers