Hmm, really not sure mate.
I guess it'd still be a car if you keep the original steering set-up, but Reliants are classed as "motor tricycles" anyway, so it's already a trike.
By removing the body you're going to expose all sorts of pedestrian-unfriendly pointy bits, (the front corners of the chassis for example), which might well fail an MOT inspection, so they'd require shrouding. Also keeping the original seat means you'll need a standard 3 point car type seat belt. It wouldn't be the 5 minute conversion you might think it'd be.
Oh, welcome to the site by the way! How did you find us?
I've got a mate in Telford -Tony Peal. He was riding a black 883 Sportster last time I saw him. We belonged to the same MCC in Berkshire back in the early 80s. If you know him, say Hi from me please.