Even in my worst estimates of Brexit, I could not have predicted this. McDonalds has run out of milkshakes and bottled drinks. This is part of a more widespread problem - truckers.
Brexit has led to a lot of truckers leaving the UK, international deliveries take a lot longer (which worsens the trucker shortage), and the pandemic has made things worse.
Various supermarkets are also having problems with deliveries, and Nandos and KFC are suffering from shortages.