I do not believe there are any speed limits of less than 20mph on
public highways in the UK. I doubt the police can specifically enforce
the lower limits that are often signed on private roads and in car
parks, although they might well be able to arrest you for something
else if they saw your fast driving as dangerous.

Breaking the speed limit is not an "arrestable" offence (unless various
other conditions cannot be met, such as being of "no fixed abode"). However,
being stopped by the police has arrested your high-speed motion :-)

--
MatSav