It's an offence to arrange for the delivery of a bladed item to residential premises.
[s.38(2), Offensive Weapons Act 2019)]
However . .
"It is a defence for a person (“the seller”) charged with an offence under section 38(2) of delivering a bladed product to residential premises to prove that—
(a) at the time the offence is alleged to have been committed, the seller had procedures in place which were likely to ensure that any bladed product delivered by the seller to residential premises would be delivered into the hands of a person aged 18 or over, and
(b) the seller took all reasonable precautions and exercised all due diligence to ensure that the product to which the charge relates would be delivered into the hands of a person aged 18 or over"
[s.40(3), ibid]
https://www.legislation.gov.uk/ukpga/2019/17/part/3/enacted
So, in your case, the seller was obliged to ensure that the chosen delivery method required age verification on the doorstep.