As far as I can tell:
If you are in any way disabled you have to prove you can ride in a possibly modified vehicle.
If you are disabled and can ride in an unmodified vehicle you may have to prove your ability to do so.
If you have a modified vehicle then you have to prove you aren't a chav.
All these proofs should be provided before you are allowed on the road, and if you are on the road you should be able to provide them when asked by officials.
(Officials under the law include: Police Officers, PCSOs', members of the BNP, passing members off the public over the age of 4 (birth certificate not required))
Look, I didn't post in blue that time....
ClickKeep passing the open windows.