I have been using digital speedos on all of my bikes for many years now. There are lots of different models, some do even fit the thicker motorcycle spokes. Some motorcycle parts sellers do sell specific magnets for fitting on thicker spokes as the use of these speedos is pretty common nowadays. I have also just glued a string magnet directly on the rim several times, works fine too.
Things to keep in mind:
– use a speedo that can indicate over 99 km/h. I guess that in fact means two digits and 99 miles/h should be enough for the avarage two speeder I guess so this might not be so much of a problem in the UK. In my experience a speedo that only does 99 km/h gets increasingly inaccurate when getting close to that number
– use a speedo with cable, not wireless as those suffer from interference from the magneto, plug leads etc
– use as strong a pick up magnet as possible (neodyme is best). The weak bicycle style magnets sometimes do not keep up with the higher rpm of motorcycle wheels
Good luck with it!