I think this has come up before. I don't like front number plates - faffy things that as Asbo Jon says get in the way. The Road Vehicles (Display of Registration Marks) Regulations 2001 are the relevant Govt bumf. Part II Section 6 (5)says:
"In the case of a motor cycle or a motor tricycle which does not have a body of a type which is characteristic of the body of a four-wheeled vehicle—
(a) a registration plate must not be fixed on the front of a vehicle if it was first registered
on or after 1st September 2001,
(b) a plate need not be fixed on the front of the vehicle if it was first registered before 1st
September 2001."
So no front plate allowed for a M3W five speeder. Depending on how you interpret the 'body of a type' clause. And a pre-2001 M3W can take its pick.
www.legislation.gov.uk/uksi/2001/561/pdfs/uksi_20010561_en.pdf
Notlob