There is absolutely no legal requirement to produce identification when buying knives. Just like there is no legal requirement to product identification for buying alcohol. I've never been asked for a driving licence when buying knives, nor do I know of anyone who has. It is illegal to sell a knife to someone over 18, but they obviously won't be asking someone who is 50 years old to product a driving license. If people suspect you are under 18, they can ask you to prove your age before selling you one, but there's no blanket requirement to prove your identity before buying.