Many nude beaches in the UK are in quite isolated spots which discourage casual gawkers, as they can't really use the "just passing" excuse. But it makes them difficult for naturists to visit also.
Brighton is the opposite, it's in the town so it's easy to access, but there's bound to be people going there just to have a look and see if any pretty girls are tanning naked (they will usually be disappointed!)
As has already been said, there is also a bit of gay exhibitionism going on there. Brighton as a town has a big gay scene, and while not every gay man using the beach is there to show off and pick up, of course there are always going to be some who are into that sort of thing.
Also it's a pebble/shale beach, not sand. This is less comfortable to lie on or walk on.
It's great that Brighton nude beach exists and it's perfectly possible to have a nice time there. But if you want a really great nude beach, Brighton isn't it.