Met someone today who is going to this
You can read the site for yourself, but the crux of the idea which is very clever is this:
"Be at T5 International Arrivals at 11am to put on (or strip to reveal) your brightly coloured ‘STOP AIRPORT EXPANSION’ t-shirt: a visible presence of public opposition to the madness of airport expansion. Wander round for a bit, have a coffee, leave when you like.... PLEASE NOTE: Almost everything is against the Heathrow byelaws, but wearing a t-shirt is not a crime. So as long as you’re not demonstrating, you’re not breaking the law!"
Should be quite big, judging by early takers on the Facebook group alone for this event. They even have a free t-shirt on offer.