Hi Stu -
Good luck on drafting your letter! Actually, what I would suggest is for you to treat the Avastin and Keytruda requests separately, in two different letters/requests.
This is because the two issues are very different and would require two different kinds of logical reasoning.
Avastin has been approved for colorectal cancer by the European Medicines Agency for a long, long time, with the most recent approval update dated August 31, 2018.
Keytruda, however, has been approved only for melanoma, NSCLC lung cancer, Hodgkin lymphoma, and urothelial cancer by the European Medicines Agency, and has never ever been approved by them for use with colorectal cancer.
(see the documents in the two links below)https://www.ema.europa.eu/medicines/human/EPAR/avastinhttps://www.ema.europa.eu/medicines/human/EPAR/keytruda
In the case of Avastin, you would be requesting to have the NICE Committee reinstate the earlier status authorizing Avastin's use in the NHS system.
In the case of Keytruda, however, you would probably have to deal with a different agency in order to first of all get Keytruda exceptionally approved for use with colorectal cancer, then deal with the NICE Committee afterwards. I think that the NICE Committee ordinarily only deals with drugs that have already been approved for a specific use by the EMA.
According to information given in the link below, it appears that the agency that can grant such exceptional regulatory approval in the UK is the MHRA (Medicines and Healthcare-products Regulatory Agency)https://www.cancerresearchuk.org/about-cancer/cancer-in-general/treatment/access-to-treatment/how-are-drugs-licensed-in-the-uk