When you tax a motorcycle in the UK, the category is 'bicycle'.
Actually, trikes form a bit of a grey area. A motorcycle to which a sidecar has been added is still a 2-wheeler. Those weird motorcycles from Yamaha and others, with 2 front wheels, are still bicycles. Something designed from the outset as a 3-wheeler, like a Reliant Robin, can be taxed as a car or a motorcycle. You may choose to register it as either, depending on what driving licence you have.
Pretty sure I've seen a TukTuk on the Island. Annual road tax £25? get in.