I really wouldn’t worry about Heathrow capacity come March 2019....
Brexit, come what May (date or person?), is most likely to result in a smaller hub (domestic) market and a potentially greater foreign (non-EU) market than ever before (I suggest). Probably equally, if not more, beneficial to EGLL, than present status. Not great for the regions....but hey ho. Everyone will need to re-adjust....perhaps using their regional (local) airport to link to a European hub instead.
Certainly shifts the CO2 emphasis, if the delays aren’t too great, connecting via Yurp...
(we obviously won’t need to worry about UK’s contribution to climate change in that case...)