It can't be a staffing issue as I ordered 2 in-stock Disney steelbooks yesterday evening and they dispatched last night (presumably in white boxes). I think Zavvi don't have this in stock yet. They do have a happy knack of sending Disney animated classic steelbooks out days before release, so Disney might have decided (I'm pretty sure like they did with Iron Man 3 and Thor: The Dark World) to not even get the stock to Zavvi until Monday. The email about posting on the 20th might be to give them a couple of days to process them and get them out.
With Disney playing hardball with Amazon, and they did it years ago with Blockbuster (they wanted £40 per rental copy of titles you could buy the same day for £15 to own) I think they're at least partly to blame.
I'll just wait. We're so used to getting things early it's become 'the norm' but every once in a while, things just happen.