Probably the for the same reason this country hasn't been able to have their agents infiltrate radical Muslim groups here. It's pretty difficult to do. First, you have to find Muslims that are willing to put themselves in that situation, and if you think it's hard to find a Muslim to speak out against the terrorists, try and find one who'll infiltrate and work as a spy. Look at the problems this country has had with Muslims in our military. Your suggestion is poor to say the least. And how do you know that the public as a whole thinks Scotland Yard is full of incompetents? That's only your feeling on the matter. It's doesn't make it so.
And your views aren't based on your "feelings" but rather pure logic and therefore true and beyond criticism.
The only thing that makes Scotland Yard's agents less laughable than Inspector Clouseau is that theirs have real guns.