Slightly different, under Use of Invalid Carriages on Highways Regulations 1988 http://www.legislation.gov.uk/uksi/1988/2268/made, S 10(1)(a) a class 3 vehicle - those with a speed limit above 4mph (which are allowed to go 8mph on roads) - must have "a device which is capable of limiting the maximum speed of the invalid carriage to 4 miles per hour". They are also required to have an indicator, which they must by the same law be able to see when using the vehicle, which shows if the limiter is in operation.

It seems a bit harder to sell than speeding a few mph above the limit in a car; but in either case those that want to flout the law are able to, you are right.