"In my borough last year over 300 children had to be offered places at a school which didnt even feature in their top 6 choices because there was not room for them in the borough schools! "
I know some people this happened to - each and every one of them used all 6 preferences on schools they had no hope of getting in to, and didn't list their nearest schools.
"You might want a GS education for your child but for whatever reason they miss out on a place or there is actually no GS close enough. "
GS's are a rare hangover, if there is no G in your area, the comps should have a fair share of GS-ability kids in the top streams. If your kid misses a place in GS for being a few points below the entry level, then your child may well be better in an area with genuine comps anyway, rather than in a high school where all the GS kids have been creamed off.
I really don't get why people are so hell bent on GSs when they are not in a full GS area.
There is a frenzy about education, and a middle class panic.
Maybe, OP, since you make such assumptions, an answer could be 'to avoid those of a hysterical disposition about schools'. I'm not saying it is, but your OP begs so many questions and makes so many quite offensive assumptions.
I know it isn't always easy, but IMO the panic grips more than it need.