I generally run at about 16 mph - i.e. just about the ebike cutoff speed.
In windless conditions on flat roads I am often at about 17 mph once I get up to speed - clearly the ebike motor helps me get there faster but other than that I mostly stay just over it - but then I probably drop down a bit and the motor helps me go back up again
On which basis if the cutoff speed was higher - I would probably go higher - and 15 mph is quite enough for a bike (IMO)
If the cutoff speed was 20 mph then people would go at 20 mph - including on shared paths, tracks and tow paths - every ebike I have had (4 to date) has encouraged me up to that speed. If I come across other cyclists on narrow paths (often tow paths) then I normally knock the assistance off in order to stay behind them easily.
And, of course, there isn;t much support for it across the country - as demonstrated by 2 recent government petitions in spite of a lot of people pushing them on the ebike forums etc
https://petition.parliament.uk/petitions/332333