orbitmic

There is no reason transit should be affected. In the UK, you currently are getting the exact same transit experience whether you are flying from the EU/non-EU to the EU-non-EU. The only exception are domestic flights.

The only potential - as well as extremely unlikely - impact of Brexit is actually a lot more dire. There is still no agreement on airline rights meaning that there is still the need for some agreement of sort of UK airlines - including BA - to still be able to fly to the EU and vice-versa. A transition period has been agreed in principle so short of an always possible major souring of the relationship between the UK and the rest of the EU, the potential risk period would likely to come after December 2020. Even then, as mentioned, the possibility of an end to traffic rights seems extremely unlikely, but at this stage and given the remaining distance between British and EU positions on many crucial aspects of a prospective deal as well as the vibe of the most radical politicians around, nothing can technically be excluded. For what it's worth, I would personally rate the chances of even a temporary suspension of traffic rights at below 1%, but then, what do I know?....