I had a thought on why they might have moved the production to the 1950s (or at least post WW2).  It's some wild speculation, but I think its fun speculation .

The change may be to provide a thematic backdrop for the story.
We have seen from the set photos, that creation of Narnia appears to be foreshadowed, but what if the shots of Digory walking past bombed out areas of London was also a foreshadow of Charn.  Charn is also a war-torn city, ultimately being destroyed by the deplorable word.

This would also link in with Digory's mother.  At the start of the story, Digory's mother's condition is not unlike the ruins of the bombed out city (with the dead Charn being the ultimate symbol of Digory's worst fear for his mother).  In contrast, the new life in Narnia (and the 'a new world, a new beginning' poster) both foreshadow what will later happen for Digory's mother.