Hi couldbeanything, I'm happy to read and respond. I hope that someone else comes along though as I don't have first hand experience of either school you mention.
An almost automatic entry to UCS prep does make this an attractive option. Do you know whether the entry to UCS senior is also almost automatic? Not having to fret over the 11+ would be a big bonus.
Stuff like travel to and from the Prep and senior school for activities sounds daunting but the staff at the school will be experienced in managing it. My daughter used to do gymnastics at the Talacre and we saw lots of pre prep and prep school children using the games fields there. If your child goes to school in centralish London, there is a good chance that they will have to do some travel for facilities.
I don't have a strong opinion on whether single sex or mixed is better. My own children go to a mixed sex school but may go to single sex schools when it is time to move to senior school. Someone else will have an opinion on this.
Lunch not being provided - is this all the way through or is it only in the pre prep? Having to provide a lunchbox every day is quite annoying but people all over the world do it. For the right school I would put up with it.
Re the pollution. I don't mean to sound dismissive on this - it is important that our children are breathing clean air. However, there are limits on what the school can do about it, other than move to another site. If a school is in a place that you believe to be very polluted then it is not the right school for you. Please don't pick a school and then go on and on about air pollution once you get there.
I don't know much about Alleyn's - it is far from where I live but I've heard people say nice things about it. Which school do you think you prefer?
Academics at school are important but are not the only thing. Pick a school that suits your family - your children are at school a long time so it is important that they (and you) are happy there.