Schools will publish a list of criteria - they have to do some things like giving priority to Looked After Children (kids in the care system, whether this is in residential or foster care), then they may have sibling policies (but as you say, it's generally not so important whether kids go to the same secondary if they're able to get there independently of you.
There isn't a catchment but as others have said, there is a distance criteria which will be used if there are more applications for a place at a school then there are places available. So distance will depend on the ratio between applications and distance.
I live in London and have sons in year 10 and year 8 at secondary. My boys both chose the local secondary school - this isn't oversubscribed though it is getting more popular and has increased its number of classes in both their years (not for all years I don't think). If it had been oversubscribed we;re only 5 minutes away from school and the next nearest co-ed (which is OFSTED outstanding, 11-16, in a working class area and has been oversubscribed in recent years) is 2 km from our house..
The other thing is that in some areas of London, oversubscription has become less of an issue - many primaries are struggling with falling school rolls and families moving and out of their area.
We also have a girls secondary school which is in a more middle class part of the borough but very accessible by bus from here and is popular with girls in this area. Unlike our local secondary schools which are all 11-16, it has a 6th form. However, it has been hit by funding cuts.
What do you think about sets rather than mixed ability teaching? If you feel your DD is less academic maybe being in a more flexible mixed ability setting with various options and paths and mixing academic/non academic options would be better for her. If she's sporty you might want to look at schools which have better facilities to suit her. For example, a friend wasn't happy with the school my kids go to for her son and moved him to another school which has a lot less outdoor space than the local one - her dd in year 7 has gone to the girls' school. The kids' school has its own gym and football pitches but has also arranged with the local sports centre only 10 minutes walk away to use their facilities for some PE lessons while doing some renovation work.
A lot of posts here about good schools are from parents who are high achievers themselves and prioritise exam results and high status. Their choices might be ideal for your younger child but not for your year 5 DD.
We live in an area of London which has a difficult reputation but we have a park across the road, 2 minutes from our house, and another 5 minutes from the house next to the boys' secondary school. And there are loads of others. For us, these are things which are really good about living here, and DS1 has lots of friends living really close - one on our street about 10 houses away!, 2 or 3 just round the corner. DS2's best mate is about 15 minutes walk away from us, 10 minutes from school, so it's easy for DP to go and pick him up (at 12 I want to encourage him towards independence but I'm not keen on him coming home alone in the dark, particularly now as evenings draw in. I think I'm actually more protective of my sons than my mum and stepdad were of me. I tear up and worry a bit about every report of knife crime, including murders of (and by) kids no older than DS1,
Anyway, what I'm saying is look at areas and schools which will suit your family and your kids, not just those which have the best image and reputation. I grew up myself in an inner city area followed by a move a mile down the road to a slightly more middle class area, but I don't think my experience of growing up in the latter was necessarily better (or worse) than the first. I didn't like our second house as much, though we probably did need the extra rooms, and I had a smaller bedroom with no chimney so couldn't have a gas fire. We had more problems with burglary in the second house, partly because the layout of the street actually made things easier for burglars than in the working class area. We had a family of very lovely neighbours who I'm still in touch with and brought a lot to my teen years, 2 doors down, but on the other side of us, we had a neighbour who really shocked me. I was being bullied at school and everyone was saying I was a lesbian - not that there is anything wrong with being one but I was the target of homophobic and other bullying, This gossip reached the ears of our neighbour who I then heard telling her DD that she shouldn't come round to play with my brother and sister on our swings because I was so "weird". How can someone be that nasty to a very shy 13 year old.?