Looked in to this a little bit, the signs are from Network Rail and we all know their stance on UAV’s as well as the way the can be.
Looking at OS mapping would suggest that there is at least one right of way going through/under the Viaduct which presents an interesting legal question as we know that flying on a public footpath is allowed so why would these be any different?
From what I understand about Network Rail’s rules, you’re not allowed within 40 metres of the railway, so interesting to see if they’re allowed to ban flying in the whole immediate area.