Gestern um 22:57 Uhr eine Antwort von British Airways auf meine Frage bekommen:
Thank you for writing to us enquiring about leaving airport in transit.
For security reasons, non-travellers cannot go through customs and passport control to meet any travelling/in-transit passengers.
As a traveller you can exit through customs and passport control to meet people or leave the airport if you have ample transfer time between flights and the necessary documentation to ‚land‘ you into that particular country, eg. any necessary transit visa and onwards flight boarding card(s).
As your flight connection is less than six hours, including time disembarking from your first flight and check-in/security clearance times for your next flight, you should not attempt to go into Central London. Any missed flights as a result of being late back to the airport will be at your own expense and time.
I hope I have been able to clarify your particular concern.
Ich hab mal gegoogelt. Es gibt einen Starbucks in „meinem“ Terminal in Heathrow. Dann kann die Wartezeit ja nicht so schlimm sein.