Vaccinate everyone. NHS can't even handle a flu outbreak. Total chaos would ensue with smallpox.
Vaccinations can easily be handled. It's the panic at even the threat that I'm concerned with. Free Republic was infested with a few hysterical newbie chicken-littles when a couple of dozen people were infected with anthrax, and they spamed every relevant thread creating fear. (And there were a lot of threads.)
If there is even a small ineffectual smallpox attack, a panicky population will drop our economy and our society to a crawl for weeks, maybe longer.