Unlike almost all other airports, you can't exit the secure area of LHR from departures, you must board a flight.
Originally Posted by
UKtravelbear
It is possible to exit every terminal though it may not be obvious on the route to follow.
At T5 BA and HAL staff escort people wanting / needing to leave departures once per hour so the proper procedudes can be undertaken and a record of you leaving airside is made.
So no you don't have to board a flight. What do you think happens to people whose flights get cancellled?
At T3 you can go from the departure hall to arrivals via a door opposite gate 23.
The picture comes from
this post in the Finnair forum.
(I haven't tried that route, but according to the LHR terminal maps, you'd take an escalator up one level to the arrivals level and walk to immigration & customs the same way you'd do if your flight arrived at T3. You could also access Flight Connections and take a transfer bus to other terminals.)