[Ed the Coastie]

There are a couple of ways that you could go with this...

Remember that the Kings and Queens of Narnia were summoned when they were needed most. I think that Aslan would counter the presence of Her Frigidity by deciding that perhaps it was time to summon Britain's own legendary defender -- namely, King Arthur.

Alternately, you could have a few of the old Norse gods decide that Jadis is really a jotun sorceress, and her actions are essentially the opening salvos of the battle of Ragnarokr. Perhaps now would be a good time to send a couple of barracks worth of valiant warriors in a spoiling action against her.

My preferred response, however, depends on how effective her magic is on Earth. Without magic, Jadis is simply an extremely charismatic adversary. With small amounts of magic at her disposal, she will likely soon be in charge -- although probably behind the scenes as the puppet-mistress pulling Hitler's strings. If she has most or all of magical powers, however, then the Allies are probably going to need magic of their own in order to counter her (see GURPS WWII: Weird War II, Chapter 3 -- The Dangerous Element). My personal preference for such a campaign would be to have the PCs working for Sir Maxwell Knight and/or the Quatuor Coronati Lodge.