An overall self reported sample of the country doesn't really bear any relation to your specific circumstances.
Your preferred school will have an admissions policy where it is made clear which group an application will be placed in and then how they will be ranked in the event of oversubscription.
The first places will be granted to those who have had the school named on an EHCP. These are, however, done outside the standard applications round - but they still form part of the published admissions number. It's a legal requirement.
Then there is usually children in local authority care (whether this is all looked after children or children from or placed with Catholic families varies) and those who have compelling reasons that have been evidenced for them to be placed in that school. Somewhere between 5%.Also a legal requirement.
Usually, next are siblings. A ballpark figure on those is around 20% of applications.
Then there are those who it sounds as though they match your circumstances. The real detail is based upon whether the admissions policy differentiates between baptised Catholic and that's it or Catholic, has taken first holy communion and attends mass weekly/other frequency as confirmed by your parish priest.
Following the Catholic applications/LACs, you'll have staff children (in some schools), other religions in assorted categories and no religion.
Once the criteria has been applied, the final tiebreak between equally ranked candidates in by distance. That's a legal requirement. Anything insinuated by other parents is nonsense.
After the offers are made, people have to accept them and decide whether they wish to be placed upon the waiting list for any higher preference schools they were not offered. Over the coming weeks, people will change their minds and withdraw (the private sector accounts for some of that), be offered places at higher preference schools in turn and it's normal for this to happen until June/July.
You will have a right to appeal if you are not offered a place in the initial round. There will be details of how to do that on the school website by NOD. Whilst waiting for the appeal, a place could come up - again, other parents could be appealing to different schools and then take up a place with them/withdraw their acceptance of a place at your preferred school.
That pretty much covers it - my point is that anecdotal accounts from random Mumsnetters is not going to help you at all.