The government should not stop people from having access to a vaccine that has long been proven to be acceptably safe, as long as they don't deplete the emergency vaccine stocks. If the vaccine would be paid for by those having it and if it comes from a non-emergency source, then I agree with you: voluntary Smallpox vaccination should be available.
Unfortunately, the gutless British government won't allow people to get vaccinated at their own expense either. Hence the only people in Britain under 30 who would be guaranteed to survive a Smallpox attack are British servicemen, citizens of a few third world countries and, ironically, Russian nationals.