What defines the markings in law is if they are a regulation 10 sign under the road traffic regulations act. Reg 10 allows the legislation to be linked to various bits of legislation that make them endorsable and require a NIP. If they are not a regulation 10 sign then they are just helpful bits of paint.
That said if you undertook another vehicle and it all went wrong they are good evidence to show that you rode off the carriageway.