Cut out mill hill, doms and watford are way better ( i have friends at all 3 schools and ive heard a lot of the good mill hill teachers moved away to a nearby private school that opened up a year or two ago). As for the choice between watford or doms base it on whether you would work better in an environment where you're allowed to roam free and treated as a college student (st doms) or whether you work better in a more structured environment where you dont get days off and stuff (watford grammar)