England premises injury: does a warning sign automatically protect the occupier?

No. The Occupiers’ Liability Act 1957 says a warning does not automatically discharge the occupier’s duty unless, in all the circumstances, it was enough to make the visitor reasonably safe. The wording, visibility, location and nature of the risk can all matter. Official source: Occupiers’ Liability Act 1957. Guide: https://getinstantlaw.com/public-place-injuries