Because secret police bureaus like that are always a good idea! 
I would rather withdraw and pump that cash into all sorts of social programs and/or tax cuts or whatever, I think they would be much more of an election winner if marketed right.
All those billions of pounds spent in Afghanistan and Iraq could easily finance a major social plan, like free public transport or something. Or research and construction of new means of energy or cleaner cars or whatever. I don't want to sound like a hippie here, but I think that as a politician you'll much sooner win over the voters with ideas that actually matter to them than vague concepts like counter-terrorism.
After all it's been a year or two-three since a real terrorist attack happened on british soil. Seems like the current prevention teams are doing a good job as it is, and I'm pretty sure the war efforts in the Middle East aren't helping to prevent terrorist attacks.
In the end, if they want to bomb shit, they will bomb shit. Heck even you and I could relatively easily make a home-made bomb and blow the crap out of a train station and kill a few people. You could also buy a rifle and shoot up a bus of people. I'm just saying...going overboard on a counter-terrorism budget isn't going to help. But maybe you just meant it as a way of politically winning over the sheeple who vote out of fear? Even then I think there are more effective ways. Unless you're planning to run for the BNP, haha.