Have a morning to spare at London-Heathrow before flight leaves at 14:30. Suggestions for a quick photo walk?

Long story short, I'm visiting England for a few days from the US. I'll be staying at London-Heathrow the night before I head out. My flight doesn't leave until 14:30. With only public transportation, are there any photogenic spots nearby that I could spend my Saturday morning at, rather than inside the airport terminal?

Windsor and Kew Gardens have been suggested, but not sure they're close enough to visit and have any time left over before I need to head back.