Tesco stores have long had such restrictions. As I understand it, the Criminal Justice Act 1998 states:
It is an offence for any person to sell to a person under the age of 16 any knife, knife blade, razor blade, axe or any other article which has a blade or is sharply pointed and which is made or adapted for causing injury to the person.
Granted, that probably doesnt include cutlery, but there is no harm in taking the precaution. I think in stores they do have the warnings posted up, but regardless the cashiers check (try buying some at self service). I think it just gets less coverage because they are not covered by the "Think 21" scheme, so few people get ID'd.