So you told them the product was not worth the value written on the box, and they probably said. Well what is done, is done. Because changing the value on the box is probably the only thing you can do, which I doubt they are going to allow you to do.
if you buy any goods on which tax was paid in another EU member country, you don't have to pay any more tax in the UK.
So you are paying UK duty?
I do not think their is any way out, but to explain the value of items is not it seems, if you could prove otherwise.
You have to understand that a nation is really a business ( or in a lesser sense a gang ( a group of indivisuals with huge sums of money and weapons stored in one place )) and you have to pay your fair share.
They gave you the option, that you did not know about. Like fine print in a contract.
A long shot thing would be to contact the person you recieve the items from, and ask them to some how revalue the items sent. That is a real long shot thing.
However that also defeats the purpose of express mail if that is how you recieved it.
Another idea, might be to send it to another nation you can hop skip across, and not pay taxes. Anyways that Queens land, and that is it.
I think in the end your going to eventually have to pay the fine.