I went to a catholic primary and then secondary school (all non fee paying)
When I went, they were fairly easy to get into you just needed to be baptised, but because of their good reputations and because of the higher percentage of eastern europeans in the UK now who are often catholic (that's not a criticism, just a fact) they are now very competitive to get into, my sister is going through this with her kids, the oldest is 8 but she's making sure they have their first confession, communion etc and are attending church regularly, as the schools often have a list of criteria of how they admit children such as
"Catholic from non-catholic family, not attending church regularly" will be below "catholic from catholic family, baptised, communion, attending church regularly"
It is a great school though, much higher rated than any of the other schools, I left with 13 A* GCSEs.
Catholic secondary schools often have intake schools from catholic primary schools, so if you know the secondary school you want her to go to, it would be worth getting her into one of the "intake" catholic primary schools.