If her height is beyond the 100th centile then surely her weight should follow the same curve?

There are obviously situations where weight becomes an issue but that's often when the weight:height ratio is not compatible.

I've no personal experience with this though, because my son has been a steady 50th percentile kid from conception to present, and the babies I work with are often the ones who are struggling to gain weight.

I think you'll probably find though now you're moving to baby lead weaning, that she's also at the age where she'll start moving more. Between now and when she starts walking she'll gain weight in some areas and lose it in others, and then when the walking comes along she might even drop a diaper size or 2 because once they start moving they just don't stop.

So unless there's a risk to her health which they've outlined (very possible if you've got a history of weight related conditions in the family, though can't think of any other reason) then it's a bit wild that they're telling you to offer water over milk.

Have they offered to revisit this issue once weaning is established? Can they refer you to a paediatric dietitian?

The industry standard is to feed on demand, as outlined by UNICEF and the WHO, and advice would only be offered in contradiction where genuine medical concern is expressed.