gscrym - you're right, it is an English term. I got confused too about and did a thread on the same topic here last Septmeber
In Scotland we don't use the term and children go straight into Primary 1. In Glasgow (and I think it is the same across Scotland), children normally start school in the August for all children who are 5 any time between the beginning of March that year and the end of February the following year. There is only one intake - unlike many places in England which seem to do an intake a term - hence reception being a wee bit confusing.
Enrollment is normally in the last week of January at your local primary school. In Glasgow, if you want to do ao placing request, you still have to enroll at the "catchment" primary and put in a request for the school you want to go to.
In theory, if your child is not 5 by the August, you could hold them back until the following year. However, I ithink once you have started, you are "into" the compulsory schooling regime and could get "done" if you take your child out of school for wahtever reason, even if they are not yet 5.
If your son is 2.5, you do need to get him enrolled at a nursery school for your free 12.5 hours nursery (if he's not already at nursery). If his birthday is September/October-ish, then he's entitiled to his place from January.
I didn't get my act together, so by the time I rang the nursery I wanted ds to go to, he could only go in from the August - but that still meant he has had a year at an excellent nursery. (And as it happens, the Glasgow nurseries were on strike from January to May, so he wouldn't have been at nursery anyway - plus he just stayed at his excellent childminder)
Ds starts school in August. Time flies