From what I can tell, the big reason that supermarkets in mainland Europe can work around this by ordering from different suppliers is that they're willing to pay their suppliers more and pass the cost increase on to their customers, whereas UK supermarkets aren't so willing to do this. That is, the main contributing factor seems to be precisely our supermarkets' unwillingness to engage in "price gouging" - and if they had put prices up, you can bet that would've been blamed on Brexit as well.