As Pete says, the rules are going to change, but don't know if they have yet. As it stands you must have owned the bike for 12 months, AND lived in the UK a couple of months when you owned it. You will pay 5% duty + 10%GST.
You can import a motorcycle built before 1989. The taxes payable to customs are 0% Duty + 10% GST
The GST is charged on the figure you paid for the bike plus all shipping charges, which are probably about $2500+ with agents fees.
Whether the rule changes, if and when they happen, allow you to bring any bike in, I don't know, but its bound to be an expensive exercise that is prohibitive. AFAIK the rule change is only to update the 1989 date to 25yo machines, no date.
There must be different rules with grey imports from Japan, but I've never heard of anyone importing a modern bike privately. There are likely to be ADR compliance reasons, too, especially if the model wasn't imported here.
You could contact Get Routed, who ship container loads of bikes for customers, and may have experience of this problem.