We were discussing this at work the oher day, and UK law is interesting. The law applies to "motor vehicles". Under UK law a motor vehicle is: "Any vehicle intended or adapted for use on the road". Under that definition, you could get some tricky arguments at court. A road bike clearly fits the definition, but a mountain bike may not. Buy thin, slick, puncture proof road tyres for your MTB however, and now you've adapted it for road use and it DOES fit. We searched to see if there have been any test cases to set a precident, but couldn't find any. I'm fairly confident on he above interpretation of UK law though, so it IS possible to get a speeding ticket on certain bikes: however, its extremely unlikely unless you really annoy the cop!