Agree with @BettySundaes
The OP seems to be in a similar situation to the one I was in back in 2019 where I was looking for good schools with a very strong music focus that could be accessed outside a distance criteria.
I ended up with a spreadsheet that looked at things like start and finish times, the route to the school, how many changes, how much walking time etc.
FWIW, if you are doing something similar OP, lots of short changes are more exhausting than one longer stint on a train or tube. Try and go for somewhere with a variety of viable routes in case of strikes/accidents etc, and where there are flexible time options after school so they can do clubs, plays, concerts etc.
DD has a long commute, but most of it is on a single train, with guaranteed seat, wifi etc which means she gets nearly all her homework done before she gets home - it would be very different if it was 3 buses and a long walk where nothing can be done.
Definitely don't go for an aptitude test entry for a subject area that your child isn't actually interested in. They almost always come with obligations. DD's school expect music scholars to take GCSE music or else they lose the scholarship, ditto sports scholars and PE, art scholars and Art etc.
You may also need to put extra effort into enabling social life - although that is rather easier in London than in rural areas.