Myrtle Avenue near RWY 27L is a nice place to spot, especially if that runway is in use for landings.
If not then it is still a reasonably good place for departures, if 09R is being used for departures of course. I've only done spotting there, so can't comment on other locations.
This site is good:
Viewing / spotting guide to London Heathrow Airport - Plane Mad
Enjoy your trip!