From s&r's link.

"    (2) On an occasion where a vehicle is being used for fire brigade, ambulance, bomb or explosive disposal, national blood service or police purposes and the observance of the requirement specified in paragraph (1) would be likely to hinder the use of that vehicle for one of those purposes then, instead of that requirement, the requirement conveyed by the sign in question shall be that the vehicle shall not proceed beyond that sign in such a manner or at such a time as to be likely to endanger any person.    "

Clear as a fookin' bell.