You can see the 27 approach at Heathrow from my daughter's kitchen if that's any help...
FWIW, I have also been questioned a couple of times whilst at Stansted. My argument is if you're in a public area and "carrying out your lawful business" (i.e. plane spotting is not illegal) then you cannot be threatened with arrest.
Never had any real problems, the airport police have got more important things to do than to haul spotters off to the pokey. Be nice to them and they'll be nice back. As for airport security staff, just a bunch of jobsworths with no real authority. They would still have to call the police to do anything to you, and the police wouldn't do it if you've done nothing wrong.