I think the tunnel is closed at the moment for pedestrians/cyclists (?) so it would be hard to get there.
You can get to T4 (I used to live in Bedfont and walk there as well) but you had to cross some bits with footpaths or sidewalks.

I think the same maybe true of T5. Sometimes I see slightly confused looking people walking around with their luggage near T5. You can get there, but you might have to ignore a few signs. One way I can think of to get to T5 is to follow the Bath Road, hang left just after the London Hong Kong restaurant towards the Thistle Hotel, and at the end on the round about take the little lane that the buses (and cyclists) take. Cross the bridge and the Perimeter Road, and head past the taxi stand into T5.