You have more options to work towards chartership with Forensic Psych than you do with Clin Psych.
The 'traditional' route is to do an accredited MSc in Forensic Psychology and then complete 'stage 2' of the diploma. This is done traditionally by securing a job as a Forensic Psychologist in Training (sometimes advertised as a Trainee Forensic Psychologist). You will be a trainee for at least 2 years but more commonly 4'ish. You will complete exemplars in core areas and these will be externally assessed.
However, you don't HAVE to have be a trainee forensic psychologist to begin stage 2 of the diploma towards chartership. Providing you can arrange appropriate supervision and are working in an appropriate setting, you can work through your examplars without being a trainee. This is less common however and probably is easier if you have a good network of people who could help you out here, i.e. give you opportunities to fulfil exemplars requirements, such as lead training courses etc.
Download the Handbook off the BPS website. It's really helpful.
I would definitely suggest you read more around the role of a forensic psychologist as it seems you're not too clear on this at the moment.
There is a lot of over lap between the work of a forensic and clinical psychologist. However, there are also elements of work that are best suited to one or the other. Personally, I believe that in areas of work dealing with offenders/victims/prospective offenders etc, it is beneficial to employ BOTH forensic and clinical psychologists as what they can offer in terms of expertise can complement each other.
There are however others that would disagree....