Ritchie,
Please take it seriously - police in the UK is very edgy nowadays. Do not be surprised if they will come now to arrest you and search your house. IT IS THAT SERIOUS! It may take up to 1-2 months to react, but osemtimes it takes much faster.
It is not the worst - if they come and arrest you, YOU ARE MOST LIKELY WILL BE CHARGED! Not immediately, but later. Do not admit it is an prohibited offensive weapon! You will be asked how often you bought the knives from the USA. Say it was the first time ,you never bought anything like this before, you did not know it was prohibited, try to find shops in the UK that sell them. If you can find it on ebay, say you bought it from British ebay, tat you did not know it would be sent from abroad.
IT IS VERY SERIOUS! If you admit, you will get prison sentence, no joke.
Knives + importation laws are so bizzare in the UK that most likely you will not find an expert who knows them both.
In any way, STAY AWAY FROM GOLDSMITH CHAMBERS solicitors and barristers, especially Hugh Blake-James. They claim to be experts in weapons laws, but they are just praying on people who don't understand the law. Hugh Blake-James put in prison people by advising them to plead guilty when they should have pleaded otherwise.
If you are charged under importation law (Customs and Excise Management Act 1979 I think), then you may get (legally) upto 10 years, but most likely more than 1 year imprisonment. However, they must prove first that you knew the item was illegal and you ordered it from abroad anyway. This is almost impossible if you have a good lawyer, but as I said before, even lawyers who claikm to be experts in this field do not understand the law properly.
However, if you are charged under Criminal Justice Act 1988, then you have no defence. The good thing about it, though, it is maximum 6 months imprisonment, so you may get away even with a community service.
Please take it seriously, take any chance to drop the charge available - get in touch with your local MP is one of the good methods. If you are a good citizen, MP can sometimes press on police to drop the charge.
Check your house that you do not have anything illegal, or what may seem illegal - police will pervert evidence anyway, remember this, some kitchen knives will become "offensive weapons", airsoft guns will become "illegal replica guns" etc.
Please take it seriously. If I were you, I would first of all take Customs to court. Yes, I know they threaten to ask you £1500 if you lose the case, but if the matter goes out of control, you will lose your good live. By taking Customs to Court you show that you did it genuinely, you did not know about prohibition.
I wish you real good luck, you will now need it.
Good solicitors to deal with this matter - Julian Young (London) and Kelcey and Hall (Bristol). Bad ones - most of them.