“On the Park roads in Bushy Park, Greenwich Park, Hyde Park (except for the Serpentine Road), The Regent’s Park, and St James’s and The Green Parks, at a speed not exceeding 30 mph, and on the Park roads in Richmond Park, at a speed not exceeding 20 mph.”
There's a good write up here.
However, nowhere is a bicycle defined as a vehicle for the purposes of many of these Acts, other than in the RTA.
It's an open point as to whether any tickets for speeding bicycles are valid or not.