if you're taking photos of aircraft coming into 27R then a great place to be is the premier inn car park on bath road, there is a nice grass bank next to the car park, otherwise the mini roundabout at the threshold is very very close. myrtle avenue for 27L is a park on the road type place.
on the other side it's busy A roads so you might have to take a long walk there. There is a spot near T5 where you can see the runway pretty well, it's on the walkway to get to the terminal I think. I've just parked on the side of the A3044 and taken photos from there.