Problem really is that in airline planning terms, there are only 9 months left to have everything resolved as flights for April 2019 are usually bookable 360 days in advance, i.e. from April 2018 (and even before on some LCCs). The resulting turn-over from those early sales is most probably factored into cash-flow planning of those airlines, so you cannot simply say "so what, this time we will open advance booking just 90 days in advance".
So the likes of Boris Johnson should stop smirking and being smug, but actually start working on the thousands of issues big and small that need to be resolved to avoid a car-crash scenario.