Gawd, it took me years to get my head around this stuff, I was ashamed to ask anyone in real life! 
I only know bits of it now, through trial and error and experience - first of all, a lot of schools will say, yes, by all means register your interest in the school - but when it comes to allocations, it won't make a hap'orth of difference to whether you are offered a place.
It just means they might remember to send you a prospectus or invitation to an open day, when your child is the right age.
You normally apply in the January before your little one will start school - and they start in September normally, in reception class, which is mostly play based learning, that is the september before they turn 5.
You can start later, ie term after fifth birthday, but most people don't.
You'll find out where you've got in at the end of March-ish. You put three choices and your reasons on the form. (often done online)
Pre school is generally from 2 to 3-4, until they start school. Mine both went from 3 and a half, bit later than average but fine.
HTH a bit!