ManOfSussex
We wunt be druv
But you have not said what their explanation is. It might not have anything whatsoever to do with Brexit.
As I'm sure you've noticed Brexit hasn't happened yet, so I would hazard a guess it's not got anything whatsoever to do with Brexit. Things like this have happened before in terms of supply issues, however I've not seen printed/prepared signs like that on the store shelves saying they've got 'problems in their supply chain bringing certain goods into the country' in there before, hence my suspicion the signs might well be part of their 'Brexit ready' preparations for next month.