It doesn't even have to be a firearm. The suspect only has to be 'armed or otherwise so dangerous that they cannot reasonably be restrained without the use of a firearm' for them to game under the Police RoE.How do you know it isn't loaded?
It could be argued that a moped meets that description if it's been used as a weapon in order to commit offences.
The risk associated with knocking one of these scrotes off of their weapon of choice, is far less than the risk towards the general public for failing to do so. Therefore the benefit significantly outweighs the risk.
And lets not forget, the suspect is knocked off the moped because they are failing to stop for police. If they did stop when required to do so they wouldn't have to be knocked off.