I took some lap timings at the criteriums round Westminster yesterday. I wasn't at the start/finish line, so my timings won't be the same as the official ones. Be that as it may, I recorded the Surrey League race completing 26 (and a bit) laps in 1-03-15. The professional race completed the same distance in 1-00-40, less than three minutes quicker.
That is perhaps a bit misleading, because the professional race started rather slowly. After 5 (and a bit) laps the Surrey League race had taken 11-20, whereas the pros took 13-20. So the Surrey League was two minutes up at that stage! Over the next twenty or so laps the pros made up almost five minutes, or almost 15 seconds a lap.
Even so, this shows how quick the Surrey League race was. Brian Tidey did brilliantly to stay with it for a few laps at the start, and John Sadler finished in the leading group, which was an amazing performance against top class opposition.
Finally, I can't believe that the circuit was only a mile long, as there was no lap under 2 minutes. The quickest I recorded was about 2-08. I'm sure I remember the Pru Tour getting an average over 30 mph on a circuit round Fleet Street and High Holborn.