Where there is a good surface, such as in a Park, cycling is usually banned.
See Booking usually necessary and often very crowded.
Needless to say these vary almost weekly.
The two most popular, apart from cycling which Britons generally don't see as a sport, more a practical form of transport, given London's transport problems} are inline skating and ice skating.
Horse Riding Running: the Serpentine Running Club in Hyde Park welcomes visitors You might also wand to look at - which is an annual run that goes past the main tourist monuments.
The commentators are always sent cakes by lady fans and spend their afternoons discussing it on radio.