I'm glad to hear there is some scare-mongering here, although the OP was genuine enough I think - looks like his dealer has misunderstood some things.

Looked up the tax rates, and found this:
Tricycles (not over 450kg unladen) 12 months, £74

I guess over 450kg it becomes a car, and needs emission figures etc. The tax weight aligns with MOT weight so there is some sense in officialdom.

50kg is do-able I think.

1930 Super Sports Aero 'The Elk'