Discuss the system with friends in London where the catchment for a decent school can be less than a mile they would tell you it’s a great system.
Having said that there are a great many less than desirable schools in London (and other large towns and cities I'm sure) and the small catchments and oversubscription mean you simply will not get a place anywhere else should you find yourself in this position. All our friends and most of our family live in London and schooling for them is a constant stress. My sister's DD (a very bright and lovely girl) has just been allocated a school way down her preference list because despite their efforts (church involvement - they are catholic) the sought after church schools are just too far, as are the other schools for which my poor niece had to sit through various selection tests. One of the schools has only a 900 metre catchment area! I feel they have had far more stress over selecting and working towards schools than we have had going through the GS process. I also feel our local upper is far more acceptable than hers so feel very fortunate to live here. I moved here from London 18 years ago well before we had children due to DH's job.... I realise now more than ever how lucky we are.