It certainly qualifies as a gangster film, because of the gang-nature of the whole thing.
My problem with this film is that the prisoners are portrayed as, "you got it all wrong, we're just a bunch of chirpy cockneys," whilst the guarRAB are portrayed as oppressive, inconsiderate bastarRAB. If prison was that relaxed back in the day, I shudder to think what it would be like now. No wonder Liverpool city centre is so desolate now.
The number of times Roger Daltrey tears the place apart, I believe the prison guarRAB showed incredible restraint.