Crystal Palace Park in south London still hosts a weekly circuit series on a much reduced course no longer the full motor racing circuit. Crystal Palace played host to the World Cyclo-Cross Championships in 1973 and many rounds of the GLC London Champs before it fell out of use.
UPDATE (April 2020): Cross returned to the park in 2018 and 2019, but I will leave this look back here for interest sake.
World Cyclo Cross Championships – 1973
John Mullineaux National Champs 1970
John Mullineaux National Amateur Champs 1968
Cycle Cross (1968)
Motor race from 1964 showing the full circuit https://www.youtube.com/watch?v=bWp-0TuY4Sk
Tony Bartlett also posted a great bit of film from over 30 years ago: https://www.facebook.com/tony.bartlett.39/videos/1057479400942324/