BigSandyBalls2015 as I said the (partial) catchment policy has only come in relatively recently (I reckon in the past five years or so).
Actually looking at Wally Boys Admissions info, it looks as if they have less of a catchment policy than the other SW London grammars:
" In rank order of performance in the Selective Eligibility Test (SET). Up to 15 places for those boys who are successful in the Selective Eligibility Test (SET) where the permanent home address* is
one of the postcodes (SM1, SM2, SM3, SM4, SM5, SM6, SM7, KT4 8 , CR0 4 , and CR4 4 – (so localish) at the time of Common Application Form (CAF) submission deadline."
Tiffin still has a very large Inner Priority catchment area, with a radius of about 10 KM but think it was introduced to stop lots of 'far away' applications from parts of London heading into Middlesex/Berkshire.
I think at the end of the day, the schools want the children to be relatively local for all sorts of reasons but well-being and forging friendships being two of the main ones.
As I said previously, I used to do that journey (roughly) years ago, as a former boyfriend lived in OP's neck of the woods. At that time of my life I was used to galavanting across London for work and play, so really didn't have an issue with time spent travelling. I just found that journey a total PITA and I was even travelling non-peak hours mainly, so always had a seat and it wasn't crowded.
I guess if a DC knows no different they would get used to it, but it's not even a direct journey without changes (which would make it more doable IMO). You've only got to have regular issues on one line for the whole school journey to go 'belly up'.
Also 1 hr 30 journeys can have a habit of morphing into much longer ones. That may not be the reality most days. DC1 goes to a school in the neighbouring borough (and we are on the right side for quick, direct access). Their journey still takes an hour door to door and that's leaving home at 7.15am.