I want to import and sell electric motor scooters/mopeds in UK (not the kids
thing, an adult, sit-on scooter). I would like to know the potential
market for this eco-friendly product in UK?

There are seriouse problem
Try this site
http://www.verteci.co.uk/legislation.asp
and also this 

3. Vehicles outside the requirements of The Electrically Assisted
Pedal Cycles Regulations 1983

Any vehicle outside the scope of The Electrically Assisted Pedal
Cycles Regulations 1983 due to the motor power output, speed up to
which power can be provided, weight, or that do not have pedals by
means of which the machine can be propelled, are considered to be
motor vehicles. They will need to be registered, licensed and taxed,
insured and the rider will need an appropriate driving license and
wear a motorcycle safety helmet.


KEMLO

Thanks - I am aware of this and the accreditation involved.