Bath Road is on the north side of LHR, so in the unlikely event that it is sunny, you will be on the wrong side. However, the end of 27 right is walkable from the Skyline - go across the Bath Road, across to the north side perimeter road, and walk along it to the end of the runway (probably total 15 minutes). The 27s are the normal runway given the prevailing winds, but you could end up being unlucky if they're using the 09s - that'd be a loooong walk. Whether 27 left or right is used for landings depends on when you visit and the time of day - the take-offs and landings swap sides at 15:00.
Many pics taken on the north side of LHR are taken from the Pink Elephant parking lot, but I think you'd need to be in a car to get in there as it isn't a walk in/walk out lot - its cars in, buses out.
Hope this helps,
There are old pilots and there are bold pilots, but there are no old bold pilots