Hi, and welcome to OpenStreetMap.

This is legally a bridleway, so why change it to a footpath?

The physical difference between highway=footway and highway=bridleway on the ground is not well defined, so arguably both are correct, but I don’t see a motivation to arbitrarily change from one to the other here.

Is this following some National Trust tagging guidelines somewhere? If so, could you please point me to them? Thanks