I'm afraid the only way for an online purchase, that eBay accept for purchasing any bladed item is to be in possession of a credit card. This proves you are over the age of 18 , not eBay's rules , but the rules in the UK.
Sorry, but the only way round this , is to add a credit card to your eBay account , or to ask someone with a credit card on their eBay account to buy this item for you.
@leelew73