Agree he could walk it, it is a possiblity but I don't believe he personally did.
Too far, long and tiring to walk.
The only people who would walk over 90- 140 minutes with a short break half way through would be hikers who enjoy walking & go on walking day trips.
Walking either of the 2 Brook House Farm routes doesn't avoid properties, people or security cameras. It goes along roads so if he was going that way he would cycle.
Bamber thought The Sea Wall route could be cycled in 15 minutes. So he's not going to spend 140+ minutes walking 8.4 miles there and back.
Walking across fields was not possible.