OP - pretending to be catholic isn't enough for most faith schools, as well as the main admissions form for the council, the faith school will have an additional form to be completed, and signed by the Priest/Church office. You might have a bit of a problem here, because they normally ask for a set length of church attendance, not just rocking up a couple of months before, so if you are only moving just before the admissions date, you might have to get a letter from your current Church comfirming that you are a regular attender to give to the new church to get them to sign the form for you.
Be warned, they often will put "regular attender of St XXXX church" as above "regular attender of another church" so you might be further down people who have been living in the area for a number of years and attending the church linked with the school, even if those people live further away than you.
If you can't get hold of the school, you could try asking at the church.
But also, have a read of the Ofsteds of all the schools in the area, go round a few, you might find that the faith school isn't the best for your DC, if you are going to Church every Sunday, get involved in the Church community, your DC will have a great grounding in your faith, they might be better being exposed to DCs from other backgrounds.
(Declaring here, we were definately able to get in the "outstanding" CofE school in our town as we have been going to the linked church since DC1 was born, however we put a closer school as a higher choice as when we visited them, we agreed the closer school would be a better match for DC1 and it was walking distance)