As someone who clerks school appeals, I come across parents at appeal hearings wondering why they didn't get the only preference they put down, but the school 5 miles away!
It's because the school only has a certain admissions number I.e 120 places, and those places have to be filled using the set, published criteria, which not everyone will fit into. So, if you don't get the only preference you put down, you will get a place at a school which is least oversubscribed with a place available. If you live in an authority which is over populated you are unlikely to get your only preference unless you live on top of the school. It is therefore advised that you put down as many preference as you are allowed.
As a parent it is your obligation to ensure your child attends school. If you don't want the child to attend the school they have been offered then it is the parents obligation to educated the child. If you choose to home ed then there are rigorous rules in place, and the authority will check you are home educating to such standards. If you choose to keep the child at home without a proper education, then the authority is well within its rights to prosecute.
The LA's obligation is to make a school place available, and once they have done that they have met their legal obligation towards the child. The rest is up to the parent.
If you do just put down one school on the application you must ensure you put your reasons for doing so on the application form. That way if you do end up in an appeal (especially infant class size) you can argue that the authority did not take your reasons into consideration when offering a school place. The Panel can then review the authorities decision I.e they had all the facts in front of them yet they didn't take them into consideration, therefore it was perverse not to offer a place - Of course, your reasons for an infant class size have to be extenuating, as the test for allowing is extremely high.
I would always always advise friends and family to put down more than one preference, as I see the results of those that don't, especially if you have more than one child and they end up at different schools, in opposite directions!
Apologies for any mistakes, using iPhone!