Heathrow Connection T5>T3, Can I use the post office?
 

I'm flying into Heathrow T5 and then connecting to T3. Will I be able to use the post office in T3?

It's located before security check in. I'm not sure if the connection transfer will bring you to a different security check in area (and therefore be unable to access the post office)

Thanks!

You can - but in this case need to clear immigration and customs at T5, proceed by Heathrow Express train or bus to T3, use the post office, and then pass through security again at T3.

The flight connections bus takes you to a different entrance at T3 and directly into a different security screening point. You never see landside departures again.