as far as legality of driving a C5 whilst drunk goes, you can be charged with "riding a pedal cycle while unfit" which states that if you are unfit through drink or drugs to the extent of being incapable of having proper control of the 'bike' in a public place then you are guilty of an offence, but you cant be arrested, but they cant breath test you so you can drive whilst 'legally drunk' as long as you are capable of driving safely, but this didnt come in until 1988, but there is also a law from 1872 about 'being in charge of any carriage, horse or cattle' in a public place, and this can be used against cyclists (apparently in case theyre not actually riding the cycle!?!). the C5 is exactly the same classifcation as a normal bike except you have to be 14 to ride it, apart from that all other restrictions fall on the makeup of the vehicle.
Lewes