There's no correlation with the destination country. The packaging simply depends on where they ship the item from.
Zavvi almost always ships steelbooks from Guernsey, and steelbooks will be in the usual white (or brown, when assembled inside out) cardboard boxes.
Sometimes they ship steelbooks from Schiedam, Netherlands, e.g., Interstellar, and then the steelbooks are shipped in A1 cardboard envelopes or slightly stronger (still very thin material) folded cardboard mailers.
If I remember correctly, they don't use the Zavvi boxes for steelbooks when shipping from their UK warehouse, either.