An invoice being just a few days over it's due date is indeed not really serious. I don't think it's a smart move on the restaurant either, if they had any intention on continuing the contract this just about kills it.
45 karma · joined August 24, 2012
edit: scratch that I was confusing css grid for flexbox, my browser does not support css grid yet.
With regard to the rules: you would still have to comply to most of these rules to be part of the single market (which most leave proponents want to be). Studies from WTO and IMF have shown that not going that route (eg trading under WTO rules) would impact UK severely (up to 9% of the GPD). See for example https://www.imf.org/external/np/ms/2016/051316.htm
Based on my own experience I would say most hobby FOSS projects ship a lot less frequently than the commercial FOSS projects.