I do not think that is supposed to be used on a public road (which that clearly is). The road is obviously used by cars, and this sign is not prohibiting cars, only bikes. Which makes no sense.
This sign does exist in the Highway Code, but I think its use is supposed to be limited to pedestrian or cycle areas that would otherwise be readily accessible by motorbikes, but cannot be used by them.
EDIT - My lad is just starting to drive, and this sign is not in the Official Highway code (just looked it up) - so it is therefore not a proper road sign and can be ignored in this instance.