Hi TFM,
There is a strong possibility that speed requirements would have been indicated on the navigational charts and in publications (Sailing Directions) covering that area. If vessels were under pilotage, then they would be certainly aware of the speed restrictions. Honestly speaking, it would be rather difficult to sight that notice regarding speed on the buoy. Needless to say, it will never be seen in the night !
Regards
vin_sun
Some reference material:- (but I have not read all of it
)
http://www.portsmouth-port.co.uk/pmsc/pm21006.htm (a particular line in this link says - Once a vessel has reached a position of within 0.5nm of the shore a speed limit of 10kn comes into force.)
http://www.qhmportsmouth.com/port-directions?action=view&id=7