If you've never been, by all means give it a go - just to tick the box if anything.
As someone who's been many times and currently uses the area as a go to due to venues being closed, Hampstead Heath can pose a few problems that may hinder your enjoyment.
1) It can be TOO dark in the evening, so much so that you can't see your own hand in front of your face in the areas where most of the cruisers gather. If you're a certified cum dump, then get stuck in, but if you're like me and like to be a bit more discerning with potential play mates, then this problem can often result in a sexless night.
2) Exhibitionism and group play kind of go hand in hand with cruising grounds, but I feel HH is one place where people will join in uninvited wherever they see guys playing even when it's clear an invite hasn't been given.
3) Drug use is prolific, more so than other cruising grounds I've used.
I'm just waiting for venues to reopen so I don't need to rely on cruising grounds, which can be fun when used occasionally, but as the main outlet for sex, it's wearing thin quickly.