Hi
I've checked the schedule and it will defiently be 27L if they are using that runway on the day. Im sure someone else can confirm but if the winds are less than 5 kt, 27L will be the runway in use. If the winds are easterly the departure runway should be 09R.
Plane-mad.com offers a spotting guide, however from my limited LHR knowledge 27L/09R is one of the trickier runways to get photos.
If 09's are in use you may get a shot from the Hatton Cross area or Caines Lane. With 27's Caines Lane will offer views of the line up, all be it through a fence and no real scope for a photo.
Im sure a LHR local will have far better ideas than this but this is guide from a non LHR local.
Thanks
Ben Leonard