From the outer ring of the M25 to Piccadilly circus in the center of this city state of London cyclists have only two places that I know of that we can train/ride at a decent pace and in relative safety, those are Regent's and Richmond and still some people still want to hobble us.
One of which, assuming you have a normal idea of "a decent pace" you'd already be braking the law whilst riding around. Its a public park, for public use. Not your own private velodrome. Frankly I think I'd probably welcome a 15mph limit in Richmond Park, would stop it (maybe) being a rat run and make it more enjoyable for the majority of other users
(not a srs request, more just to point out that that reading of the law is up for interpretation, again, FWIW in that specific instance I would be quite happy for it to be applied to push bikes)
One of which, assuming you have a normal idea of "a decent pace" you'd already be braking the law whilst riding around. Its a public park, for public use. Not your own private velodrome. Frankly I think I'd probably welcome a 15mph limit in Richmond Park, would stop it (maybe) being a rat run and make it more enjoyable for the majority of other users