I think that parents who live close to Purley Oaks will usually try to aim for the 3 very good schools in Sanderstead (Gresham, Ridgeway, Attwood) or one of the church options.
I think that you need to consider what sort of environment you want for your son. I'm clearly missing some of your other threads, but it seems that religion is very important to you. But that it simply one aspect of school life. You have a month or so left of this term - I would look around each of your 3 closest schools incl Parish and see what you like. If nothing else it will give you an opportunity to consider what it is that you are looking for in a school rather than merely the absence of religion. For example the adult:child ratio varies significantly ime, as does the amount of times outdoors, the amount of outdoor space, the turnover of staff and pupils (though obv you need to ask this), the breadth and depth of activities available, the age, gender and ethnicity of the teachers as well as their experience and enthusiasm. FWIW I found it to be a very hard decision to make on paper but much easier when I went to look around the schools.
IMHO school should be somewhere that enspires children to learn rather than just forces in information. You know your ds better than anyone - have a look at the environments and see which one inspires you. I would look at Parish as well if only because if you do see things that you like about it some of us here might be able to point you towards similar schools with those particular attributes (and hopefully without the CofE aspect). Dare I say it it might even have some positive aspects that, given the selection available, might mean that you could live with the CofE aspect.
I can say that without fear that I'm putting you into a dilemma because I think that one of the generally shared views is that the church schools in Croydon do well because they attract the right sort of parent - not that the parents are religious, but that they are motivated to get into the right school and want to ensure that their children thrive at school. Generally the keen and interested parent will make up for any lack on the part of the school. So other schools may in fact be better schools with more inspiring teachers etc, but Parish and its ilk appear to "do better" on the back of the strength of parental involvement.
In terms of ds being advanced, the main thing to think about here is what his age is. If he is a Sept/Oct born then you do need to consider how he will be accommodated within the school. If on the other hand he is summerborn then whilst he may be advanced he may not be more advanced that the Sept/Oct born ones in the same class - if you see what I mean?