In my London Borough the criteria is exactly as frogs says and its very strict. I cant find the guidance booklet at the moment, but I'm fairly sure it said something along the lines of don't bother trying to tell us how wonderful your child is etc etc - the criteria is the criteria and thats it.
Do as others suggest and contact the LEA. Ours uses a specific mapping system which can be checked out on the internet so you can see where you are in relation to the catchment areas.
If you do want to add something to the form, focus on the criteria, so if where you want to go is the closest and you are in its catchment area, then say so.
We are very close to the "top" primary school in our borough - next door is in its catchment area, but we are not. The pressure for places is enormous and we were really panicking when we realised we had no chance of getting our two in.
However, a much closer school which at first I thought was a real "second best" is I think, just as good. I did my homework and found out that since its last OFSTED report in 2000 (which wasnt bad, but nothing like the "supa dupa" school) it has dramatically improved, has a new Head and the SAME KS2 SAT results as supa dupa school last year. Its also an all round warmer, friendlier, more culturally diverse place than the one we were originally desperate to get into.
Found out today that ds1 has been offered a nursery place there for Sept and I am thrilled to bits and his chances of getting a reception place are miles better.
I suppose what I am trying to say is that all is not as it seems when looking at potential schools.