I know you can ride at Shoebury - but if you decide to - check (and check again) the tide tables, don't go alone and preferably take someone local who knows the seafront well!
Not many in Essex. Shoebury is not so much a beach but more mud flats. Never been myself but hear the best pace to go is Hunstanton, Norfolk. Beach ride is something i really want to do, but never seem to get round to!!