Green’s Dictionary of Slang

heavy mob n.

also heavy squad
[heavy adj. (4b)]
[1950s+] (UK Und.)

1. a gang of thugs.

2. physically tough police or prison officers used in violent situations.

3. (also heavies) officers from the Flying Squad and, formerly, the Special Patrol Group; thus heavy, the car they drive (see cit. 1989).