Thanks ealingwest. Sport was a crucial part of my dyslexic DD's school life, and is likely to continue to be through University. She has just finished but over her secondary school years she managed to be in school teams for seven different sports (including boys cricket!).
I think it is worth taking a step back and asking what you want from sport. If you want a shot at an elite sport you probably need to pick your sport quite early. And you may be better off selecting a school that does not emphasise sport, but is close to training, or indeed is the school whose facilities the sports club uses. Sporty schools (KGS is one example) expect their team members to turn up on Saturday mornings, and you don't want to be choosing between school netball and, say, gymnastics training. Similarly some well regarded academic schools will allow, say, tennis players training at Roehampton to be on reduced schooling, others not.
For most the bigger decision is between a school that is successful in teams sports and those that encourage participation. One surprise, particularly for girls coming from state schools, is just how good other kids are. So for a spot on the swim team, enjoying swimming probably won't be enough if four others are qualifying for county championships. And prep school girls will often have had at least four years of school team netball, so if the emphasis is on results, taller prep school girls get the spots.
American Universities expect applicants to have been engaged in a wide variety of activities including sport, and being, say, a competent hockey player might not get you a scholarship but could put you ahead of the competition for an over-subscribed Ivy. The good thing is that this means that a good core of Central London kids will keep up sport through their secondary years (with parents on the sidelines trying to capture the best moments on video to send off with the college application) but it also means that standards are high and competition is tough.
DD switched at sixth form to a school which has a relatively poor sporting reputation but where participation was encouraged. She chose to try a couple of new sports, both of which she enjoyed. She liked the fact that team membership was broader and not limited to the same sporty girls, and that training was fun even if some had limited talent. Though DD never reached that level, we observed a few kids on national pathways who seemed to have forgotten that sport can be fun. Similarly playing in a club senior ladies team allowed her to meet supportive role models who saw running around a cold sports field a couple of times a week as a natural way to let off steam.