And if you take matters into your own hands, the police will be on you in a second.

I can just imagine your swift trial:
Judge: "You are charged with the severe beating of and theft of [$800 / laptop] from [Oxford fraud / East London thief]. What do you say in your defense???"
You: "I brought him to justice."
Judge: "Leave that to THE POLICE! You're being fined, and jailed, and sued for damages and theft by this poor assaulted citizen and..." etc etc etc.

Remember, that's your tax payer dollar at work. Do you agree with how it's being used?

Honestly, though, the police are probably constrained by standard operating procedures, red tape, and bureaucracy when trying to pursue a criminal. Warrants, court orders, probable cause, "criminal's rights", burden of proof, evidence, etc.

On the flip side, I wonder if the police could be sued for professional negligence (especially regarding this thread). They simply didn't do their job (it sounds like). But, the police are so connected, they could never be held accountable. Again, your tax dollars (pounds, euros, whatever) at work. It's time for change.