I have been once and I found that Myrtle Avenue was the best that I could find!
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Sorry for the late reply only new to this site but just for next time you go to heathrow there is a BP garage on the ring road at heathrow usually when there taking off over the M25 you can snap them up there you would have to wait until they are above the fence but still get some great picts. The staff at the BP are happy for you to park up for free as long as you buy something first i usually get a drink then walk over to the grass area by the entrance to the garage.
hope this helps