De acordo com postoffice.co.uk , os correios em Heathrow ficam no terminal 3 e seu endereço é
Heathrow Airport Terminal Three
Terminal Three
Hounslow
Middlesex
TW6 1QG
No entanto, na página em poste restante (entrega geral a.k.a.), eles dizem
For Foreign Travellers in the UK: Before you travel to the UK, write to or call the Post Office to request the address details for the post office that you would like to use in the UK town where you will be staying.
For Domestic travellers in the UK: Do this in person at the branch you’d like your post sent to. Please note, the Poste Restante service may not be available everywhere.