You'll more than likely find them in cul-de-sacs. In Gt Manc there's lids with just 'Water' on, we use them too. It can be a ba**ard finding them and sometimes you'll see a 'WO' lid but there will be an 'H' yellow hydrant plate on a wall near it indicating it's an FH. Go off the plate and not the lid.
Therefore, vice versa. If there's an FH lid but no plate you'll get away with using it.