rough radius of earth 6378km at equator. At 51.5 N I make it 3971 km. So circumference at equator roughly 40074 km, circumference at london 24951km. each of these distances are covered once in 24h,. so apparent linear speed at equator is 1669km/h and at london 1039 km/h. All based on that original radius figure, which I grabbed off the net so claim no accuracy for it.