I am in two minds about this. (As indeed, I suspect, are a lot of people) On one hand I don't think it is going to do wonders for the Environment, or indeed the wellbeing of local communities. That said, Heathrow is one of the most well-known and popular airports in one of the most popular and powerful countries in the World. Those who are very much against the expansion must put forward viable alternatives of transport at International level.
As for John McDonnell, one cannot help but wonder if this was pre-orchestrated and therefore an act of indulgence and not helpful at all. It won't help his cause and only put up the backs of MP's of varying opinions on this matter.