These were already mapped as highway=footway, which is implicitly bicycle=no. Adding bicycle=dismount will have no effect on OSM-based routers, which weren't likely to be sending bicycles along these paths anyway.

It can be helpful to tag footways as bicycle=no where it is explicitly signed with TSRGD diagram 951 signs (Riding of pedal cycles prohibited). In these cases, the signs can also be added as a node on the way nearest to the sign's physical location, tagged as traffic_sign=GB:951 + bicycle=no

You may find this useful:
https://wiki.openstreetmap.org/wiki/OSM_tags_for_routing/Access_restrictions#United_Kingdom