NICE rules, however. I remember when Tamiflu was off the list.
If they decide that a treatment will not be offered, is it not true that it is illegal to purchase it privately?
No - what I suspect you're thinking of is that once you've opted for private care for a particular condition, the NHS won't then fund part of that particular care package. However, that rule seems to be on the way out, as reported here:
http://www.telegraph.co.uk/health/3255806/NHS-patients-will-be-allowed-to-pay-for-private-top-up-care-says-Alan-Johnson.html
(Sorry - can't seem to make this show as a link, you may need to cut & paste the url...)