Remember Normski on here. he tried to do it in 24 hours on his long term 1971 TR6, it broke down in Devon so he changed bikes and used a new one. I think he got there just a little late. Raising money for a charity supporting his Daughters illness.
His plan was using A type roads where he could and would have needed to keep around 60 mph most of the time. I would love to do it but having done 400 mile in a day when much younger, i would never do it now.