With an MBA in management and CGP 2.6, can I do a PhD in the field of forensic psychology?

From management to forensic psychology? That's a leap if you have not done any courses in psychology. I wouldn't think this would be possible without having an MSc to bridge the two. You'd be competing with more qualified students for the PhD unless you self-fund, and if you go that route I think it'll be a massive learning curve that you'd need a supervisor on board with to get you the right courses and training programmes.