Originally Posted by
apollo00
Heathrow say there are postboxes before and after security at each terminal but I've never found them personally (on, I've never looked very hard either). If you want a post office, you're going to T3 unfortunately (the only one at any Heathrow terminal).
In T2 Departures the postbox is on the upper level opposite Giraffe and WHSmith at the far left hand side as you exit Security.
In T3 your options are the post office (if it's open), or otherwise immediately after Security - before the duty free.
The post box in T5 Departures is adjacent to the large Travelex desk, just before Fortnum & Mason and Galleries South. Theres also one behind the checkin desks but I cannot for the life of me remember where!