When Joey Stickley tips off Regan about the robbery in the opening scene, he says that the robbers will be in a Bedford van. However we then see him in a Ford Transit van - it is the members of The Sweeney who are in a Bedford van.
Quirk is supposedly set up by being fed false information through and informant, but at the end it turns out that the informant is - like everyone else in the supposed gang - a member of the Serious Crime Squad. It is inconceivable that Quirk would not have known who he was dealing with or that his informant would not have revealed to Quirk that he was a policemen.