A Moped, by law, is a geared or automatic bike that does not have an engine size larger than 49cc (often called a 50) and cannot do more than 30mph.
When you are 16 you can ride a moped. When you pass a UK car test then you automatically get given a full moped licence (shown as category "P" on your licence). This means that you can ride a moped without L-plates and you can take a passenger. However, if you have only passed your car licence since February 2002 then you will have to first complete a CBT course.
FAQ
1/. If I put a race can on my moped and it now does 45mph what does that mean?
It means that it is no longer a moped, and potentially you violate your insurance and licence. This could be a serious problem in the event of an accident.
2/. So an 80cc moped (like a Honda Vision 80 or Honda Cub 80) is not a moped?
No, anything above a 49cc bike is considered to be a motorcycle?
3/. So what is a scooter then?
A scooter is a generic term for an automatic motorcycle. So all the laws that apply to motorcycles apply to scooters.
4/. Can I take a test on a moped?
Yes you can; but you will need to a CBT and Theory Test first. In practice very few people do this as the cost is the same more or less as it is to take a Restricted Licence.
5/. If I do my CBT on a moped will I be able to ride a motorcycle when I am 17?
Yes, once you are 17 you can ride up to 125cc on the same CBT certificate (as the certificate lasts for two years).
6/. Why is the law so stupidly complicated?
Good question; in part because things are rarely retrospective so layer upon layer of legislation is added to existing legislation, but also because very little imagination has been used in its application. Suffice to say that for much of the time even the DVLA don't know what they are talking about.