Details. So for starters, the EU currently regulates a lot of what may be sold as food, regulations that just no longer apply if UK leaves. So they have to go t

Details. So for starters, the EU currently regulates a lot of what may be sold as food, regulations that just no longer apply if UK leaves. So they have to go through all the EU regulations first just to determine which are necessary to keep safe food in British supermarkets. Then you get the international problems directly connected to that, what can you import into Britain and at which tariffs. Plus, how do you cooperate between air traffic control across the channel and across the Irish sea. (While we are at it, is British Airways allowed to fly over France, without EU law?) And that is just the things Britain has to figure out on their own. Ireland, France and the rest of the EU have interest in British tariffs and if Britain sets tariffs unilaterally they will respond in kind, a situation sometimes known as trade war. So yes, in principle Britain could just state a position on terms of trade, a hard Brexit, but that would mean that the EU responds in kind. Neither the EU nor Britain wants to do that, but the thing is the EU is something like 10 times as big as Britain and so the EU can very comfortably wait for Britain, because Brexit only affects something less than 10% of the EU trade, not over half of all trade as for Britain.