Schools are funded per child on the roll. Catholic schools - or any other schools selecting on faith to educate children on premises they own outright - are not soaking up state funding.
School planning - the provision of schools for a local area overall, and identification of need (one of the criteria for new schools) - is carried out on the basis of all state-funded schools in an area. Faith schools are included in this. So yes, a concentration of faith schools in a particular area for historical reasons, where the local demographics no longer match the faith school provision, means there is going to be limited scope for new schools. Even where there is demonstrated local need it is incredibly difficult to open a new school.
And if you want all children to have the same number of schools available to them regardless of faith, that is a reasonable position. I don agree with it, because I think educating your child within your faith is a fundamental right, but I accept that that is your argument.
It is not a fundamental right to have the taxpayer pick up the bill for your choice to educate your child within your faith. Your faith is given a privileged position in this country, and further you have had the luck to live somewhere where there is a school that caters to your faith. Plenty of people of faith do not get this provision from the state.
People of no faith are, of course, not allowed secular schools. And it is banned to open new ones.
In many countries faith education is funded by the faith or the parents. In very few places do they expect the state to pick up the bill, or for their schools to be included in state education provision.
In that case, the government would need to buy out the sites currently provided by the Catholic Church at their commercial value.
Well, not to ditch faith-based selection. That’s a simple change in the law, and it would be up to the Church of England and the Roman Catholic Church to decide whether they wanted to remain in the state system, or whether they wanted to withdraw from the state system and educate those children at their own or parents’ expense. It would be interesting to see. Given a significant number of more rural faith schools are undersubscribed, those schools would carry on as they are now.
However if the state did decide to make a generational investment in equality (which I’d be up for, because of the social benefits), I imagine any honest organisation would reflect in the sale price the vast sums of money they have saved in 70 years’ worth of maintenance, upkeep and new building. 
The government doesn't own them at the moment so no, they are not "the local schools"
The government treats them as if they are, to the detriment of children not of that faith.