If you do the Hatton Cross stop on the way to T5, it might be worth crossing over the road from the station to see the BA planes in the maintenance areas parked close to the security fence on Eastchurch Road. From the station cross over the Southern Perimeter Road then cross over Viscount Way and walk down Eastchurch Road, they'll be on the left hand side. Usually a couple of widebodies there. It's a 5 minute walk. The Southern Perimeter Road can be busy though, just be aware.