If living in the London Borough of Camden, as opposed to just wanting to send their DC to school there or in the vicinity, the OP would need to have his / her DC in primary school in the London Borough of Islington to have anything other than zero chance of a place at Dame Alice Owen. The school will not permit anyone to register for the entrance test for its selective places unless they live in the defined local part of the catchment area or the DC live in / attend primary school in Islington.
@Ib1234567 for state schools, you need to look at the admissions policy for schools in which you are interested, plus the admission statistics for recent years. No point in getting your hopes up for e school with the Mumsnet Seal of Approval, only to find when the time comes that your DC isn't ranked anywhere near far enough up the list of oversubscription criteria for the relevant year of entry to be offered a place at any of them.
If your DC are still young enough, of course, you have time to move to to somewhere with the best chance of getting a place at a favoured state school by the deadline for that year's admissions. (Or any other deadline that the school sets - you need to check).
Indies, rather different; do well in the exam, don't be rude to anyone on interview day and pay the deposit, and you're in.
N.B. if you are asking because you need a place / places for DC who is / are secondary age now, where they can get a place immediately in the state sector will depend on where actually has a place in the year group(s) to offer.