I think the reasons are because the pools are run by a midwife unit very limited on what pain relief you can use etc when doing a water birth.
They normally only take ladies in who are likley to not have any problems in natural birth only if problems arise i think they need to take you to the normal maternity unit where there are consultants at hand and more pain relief options.. (this was my understanding anyway)
I would like one, and the midwife at my hospital told me it depends on how your labour starts, if its natural and baby in right position and no medical issues with mum then a water birth is an option.. If baby need inducing or you have some issues that the consultant wants to keep an eye on then a water birth (midwife lead) isnt a option .. what they class as medical reasons i am unsure if this could be weight or not, worth asking your midwife though.
I think it all depends on how labour is on the actual day hun before they can decide if you are accepted or not for this option if things go nice to plan then i dont think weight is an issue but i suppose its the hospital who decide
x