No CX knowledge but in general, yep there’s a few fun bits here and there – as stated Sandy Heath is the obvious bit with some bombholey type areas. I suspect there are bits elsewhere that I never found, I believe Mr “esher shore” of this parish would be the go to man for information.
I rode Sandy Heath for three years and was probably lucky as no one ever gave me any hassle. I got shouted at once by a grumpy cow when I was descending at some speed from the top of parliament hill. I wasn’t anywhere near a path or any people but she seemed to think my tyres were going to cause untold damage to her precious bone dry grass.
Vaguely in the area, cycling is allowed in Queens Wood (but not Highgate Wood) and there is the Parkland Walk disused railway which might be useful for linking bits up. Everyone seems to know the bit between Finsbury Park and Highgate but there’s another section up by Ally Pally.