“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.”
http://www.legislation.gov.uk/uksi/2004/1308/made?view=plain
2004 Amendment:
“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.”