Found these items about footpaths:
"Your legal right is to pass and repass along the way. You may stop to rest or admire the view, or to consume refreshments, providing you stay on the path and do not cause an obstruction."
"A person who strays from a right of way, or uses it other than for passing and repassing commits trespass against the landowner."
"Closure and diversion - that is, a change to a path's route - can only be carried out by local authorities or central government."
"Path closures. A highway authority can make an order to close a path if it considers the path is no longer needed for public use. A notice must be published in a local paper and also placed at both ends of the path. At least 28 days must be allowed for objections."
My reading of all this is:
Unless they've applied to close the paths and published advance notices, they can't just do it on the day
You can walk back and forth along the paths, and even stop on the paths to "admire the view" which in this case will be of airplanes
If you stray from the paths then you could be setting yourself up for trouble