Well, the letter assumes that there will be still flights between the UK and the EU. And personally, i don't see how that is legally possible. Leaving the EU and all its treaties means leaving the ECAA, and of course all other open sky agreements that the EU has negotiated, the most important one the EU/US/Canada agreement. Without the ECAA rights for the UK, all but the first 2 freedoms of the air will not be possible for UK airlines in the EU and for all EU airlines in the UK, all those traffic rights will have to be negotiated in that case. As we know if there is a no deal brexit coming some time before brexit day, there is of course the possibility to negotiate a very basic agreement on aviation, which is probably one of the more pressing topics.