As far as I'm aware, a 'Road Closed' sign comprising white letters on a blue background has the same legal authority as a policeman standing there with his arm raised. Where the sign has white letters on a red background, it can only be legally effective if a Traffic Restriction Order has been applied for and granted, e.g. by the local Council, and notice of the closure given in a local newspaper. If a TRO is in force, passing that sign is an offence under Road Traffic Acts of 1984 and 1988. With no TRO, there's no offence. Mind you, knowing the plod, they'd probably opt for the 'without due care and attention' route.