Restricting a 600

[Lawrence]

There must be some kind of law involved otherwise people simply wouldn't need to, or even bother, restricting their bikes?  :rolleyes

Of course there is.  It's not driving in accordance with your licence.

Quick google and I found this.  They say up to  £1000 fine, 3-6 points and potentially a ban.  The relevant part is "you do not have the relevant licence for that class of vehicle."  You've also got to remember that your insurance will be invalid.  If you smashed up someones car , your insurance would pay out the third party (they're legally obliged to afaik) but then they'd come after you to get their money back.