To be a pathologist of any variety you need to do a medical degree, so you need to do the work experience required to get on a medical degree. However, this wouldn't be until you're in 6th form, as work experience done during your GCSEs isn't going to normally be suitable/acceptable for when you apply to medical school as it will be "out of date".
Also worth noting that to become a forensic pathologist, as noted you need to do an entire medical degree covering the full range of medicine, then complete the foundation programme rotating through different areas of medicine, then do the initial stage of histopathology training, and only then enter into higher specialty training in forensic pathology. So you'll be doing at least 7 to 8 years of the whole range of medicine (degree then foundation programme) then I think 3 years at least of histopathology training across general histopathology areas, before that specialty. So in general you shouldn't be planning on a specific medical specialty before beginning the medical degree.