Is it just poorly worded or does it really mean that you ‘must not do this, but if you do...’ ?
And what does the last sentence mean in the context of the preceding?
Okay, so I went to the Gov site to read what is actually written and that makes far more sense:
These small vehicles travel at a maximum speed of 8 mph (12 km/h). On a dual carriageway where the speed limit exceeds 50 mph (80 km/h) they MUST have a flashing amber beacon, but on other roads you may not have that advance warning (see Rules 36 to 46 inclusive).