Yes there is an airport post office as well as various post boxes throughout the airport.
See the related link for the location of each one.
Heathrow Airport is so big that it has many post codes. The post code for most of Terminal 4 is TW6 3XZ
Post office.Postal service.
Waterside. Near Heathrow
The UK doesn't use ZIP codes, it uses post codes which are much more detailed, so each of the five terminals at Heathrow will have its own post code.
Post Offices
FPO = Fleet post office for Navy personel. Apo = Army post office for Army personel
Main Post office (Head Post Office) to Main Post Office (Head Post Office)
Gumtree and Regus offer office space in Heathrow. Rightmove also offers commercial property. Gumtree is the most popular and so its options are often in high demand.
Postal service
The plural of Post Office is Post Offices.
Will be like a Master post Office
Information about post office guide.