There are some on the forum that have done this a bunch and they can speak to this better than i can. But, this is my experience. First, i live in the US. And, it sounds like the rules can change quite a bit depending on where one lives. So, you might be able to find some UK specific info somewhere.
I wanted to order a knife directly from a hamono because the price (for the knife) was pretty sweet. I figured i had to get spanked somewhere, so i looked up duties on imports into the US. I found a free calculator online because i didnt want to pay or look through the million page govt document. I entered all info into the calculator (product type, cost, shipping...) and it spewed out that there is no duty on a kitchen knife under 200$ in the US. I took the internet calculator with a grain of salt. I figured 30$ duty would still make it a good buy. It arrived friday (5 days later...expecting 10!). No extra duty.
Previously, I had ordered a hankotsu (<200$) from JCK and did not pay duty.
So, I'm loosely going with the 200$ rule in the US.
I'm not sure how an exporter can circumvent duties without misrepresenting cost/type of product. And, there are some who can make a living out of finding loopholes (which is aaalright, in my book). I just factor in an extra 20-30$ of unforeseens with international orders when US vendors don't have what I want.
Presumably, your order was over the UK version of 200USD. or maybe your knives saw the wrong (or right...depending on your viewpoint) agent at point of entry.
Maybe someone else can chime in with the "science" behind importing.