seenbutnotheard - I think you need to dig a little further into the admission arrangements for the primaries linked to Becket Keys.
If you look at this
The linked primaries are -
Larchwood - PAN 30, non-faith school
St Paul's Bentley - PAN 30, faith applicants prioritised, but non-faith pupils admitted in 2011
St Peter's CE Primary - PAN 45, of which only 10 are reserved for faith applicants
St Thomas of Canterbury - PAN 75, faith applicants prioritised, but non-faith pupils admitted 2011
Not living in the area, it's very hard to guess as to how many of the places at the primaries end up going to churchgoers. I suspect that there is a significant number of children who have got in purely on distance. Which is probably why the admission criteria for Becket Keys were approved. I doubt such criteria would be deemed acceptable if a Catholic academy were set up in Richmond due to the fact that most of the primaries are fully subscribed with faith applicants and it wouldn't be in the spirit of the law.