Good and interesting comments above. I certainly would prefer to stay legal, and a more free running bike such as a cross style with narrower tyres would definitely help keep the speed up and energy levels down when above the cutoff. The Typhoon wasn't that bike of course, so I couldn't really do that sort of comparison here. However the experiment did show me that a pedelec is realistic for the commute so something like the KTM Macina Cross 9 that Col mentions earlier would be exactly the sort of thing.
JohnCade, interesting to hear your legal Kalkhoff has a higher cutoff point. Rather than asking in the middle of this thread perhaps I should start another one to find out about perceived cutoff points, as that could be quite significant for my purposes.
D8ve, a recumbent would be great. I have looked into them quite considerably but unfortunately whilst I may be keen my wife is rather less so! I know all the anecdotal evidence points to them being just as safe as a traditional bike (or even safer) but there we go. And actually although much of my commute isn't plagued by traffic there is a bit of filtering involved on one of the routes.
Michael