When snack is a more-or-less regularly scheduled food break, a school kid, or a least a school kid in New York City, will ask a classmate "What did you bring for snack?"
Same in London. It's being used as if it were a meal or mealtime, as panjandrum pointed out, because it is eaten at the same time everyday, morning playtime/ break / snack (time).
But if you are thinking of eating something other than a meal at any time you feel hungry, then you talk about "
a snack". " We'll have a snack for lunch ( or
"have a snack lunch" ) today "
HG