It depends which route you'd want to go down. There's two broad ways of becoming a qualified forensic psychologist, one via university and one via the BPS. If you chose the BPS route (Stage 2), you'd need a masters in Forensic Psychology but the first year of professional doctorate (university led, three years total) is equivalent to the FP masters so you wouldn't necessarily need the masters before applying.
Some universities offer a top-up course which is essentially the doctorate but without the first year but these aren't so popular. Most people end up doing the masters anyway as it opens up both routes to qualification and getting onto the Stage 2/doctorate is very competitive. You would also need to be in a role directly working with an offender population, such as an assistant psychologist or interventions facilitator.