Forensic Psychology is a very very difficult and competitive field to get in to. I would advise you to do your research but also consider back up options. I had a careers talk by a forensic psychologist who has a position in a prison. She said they have cut jobs so that there is now only one per prison and you are basically waiting for someone to retire/die/quit. Harsh but (unfortunately) true.
Also, all BPS courses require you to study in a scientific manner. There is absolutely no avoiding it (unless you go for a non accredited course). If you are struggling with science (biology in particular) you may well find yourself struggling with a good chunk of the course. If, however, you enjoy sciences but just have a lack of experience with them then you may well be fine. Motivation is a very big part of success at Uni so enjoyment of studying is key.