The problem is the Offensive Weapons Act 2019 and how various couriers choose to interpret it. In non legalese, if you send a sharp or pointy thing by post or courier you must undertake to ensure that the person you're sending to is over 18. The tricky part is that the service you use to send it, your "agent", must undertake to ensure that the package is delivered "into the hands of" an over 18, which they mean literally.
Some (many) couriers won't take on the potential hassle.