Unfortunately, though cannabis is available on prescription for cancer-related issues such as nausea resulting from chemotherapy, it is still regarded by the UK government as unproven as safe or efficacious as a cancer treatment itself. To me this is one of the biggest tragedies of the War on Drugs. I get the pleasure of seeing testimonials from patients and therapists from all over the world, from people who have reduced or rid themselves, with or without adjunct-therapy. With a history of cancer myself, my last tumour disappeared when I was taking cannabis oil, then illegally. It was actually trial and error and for a while, what I could get that taught me that cannabis as a high THC flower works really well to manage my neurological symptoms too. However, because of the cancer exclusion from the prescription system means that there's no UK guidance on dosages to treat or prevent cancer and no means of obtaining cannabis oil legally for this purpose. Indeed the 1939 Cancer Act stifles any advice by manufacturers, by forbidding 'false claims'.
Further Reading
Legislation.gov.uk: Cancer Act 1939 (accessed 5th September 2022)
Wikipedia: Cancer Act 1939 (updated 1st March 2021)