The biggest charge is usually VAT rather than duty. HMRC won't care about any foreign sales tax you may already have paid; if you haven't shopped tax-free or claimed a sales tax refund they'll be happy to charge you again when you are checked on arrival in the UK or declare the goods. UK import VAT is payable on the total value if what you bring in is worth more than £390. Duty may also be payable over that threshold, but the rules are a bit more complicated.
Yes, you're right. I should know this, as many years ago I bought a didgeridoo from an Australian company late at night (UK time) and it was a question of what drunk me bought sober me. The following morning at about 11 am I received a phone call from a very nice-sounding Australian lady, she said (pretend you can hear the accent), "Gooday, Mr H, your didgeridoo is packed and ready to ship."
Didgeridoo, what didgeridoo...ah, wait. Oops.
Anyway, long story short. It made it to a courier in this country whereupon the company asked me to pay import duty and VAT at (I think) 20%. Now the Aussie company said, if I was asked, to declare it as an Aboriginal artefact which would attract 5% VAT. I tried this but the conversation with the VAT office went something like this:
"Hello, HMRC how can I help you?"
"Hello, I am being charged 20% VAT on a didgeridoo which is an Aboriginal artefact so should attract 5% VAT."
"It's a musical instrument and that's 20%."
"But it was made by Aborigines, they go out into the bush, find fallen trees, -- in this case an an Ironwood tree-- then bring them back to a workshop and, using traditional methods, fashion the wood into a didgeridoo."
"It makes a note."
"It only makes one note."
"But you agree, it makes a note?"
"I could make a note by banging a dustbin lid, would that make it a musical instrument?"
"But Aborigines don't make dustbin lids."
"They might, if they made a nice note."
"Dustbin lids don't make nice notes."
"Haven't you heard a steel band?"
"They aren't dustbin lids."
"They could be."
"It's a musical instrument, it's 20%."
"What if I don't pay?"
"You don't get it."
"You're not going to back down on this, are you?"
"Nope."
"OK, send the bill."
At which point, I gave up.
The worst thing is that they charged me import duty on the didge then there was the cost of the didge then there was the shipping (and they estimated the cost of shipping between the bond warehouse and my home and added that), so I ended up paying VAT on the lot!!!
The didgeridoo cost me £140 but by the time I had it in my sticky mitts, it had cost me nearer £230.
Needless to say, I never bought another didgeridoo from Australia...and I try to never click "BUY NOW" late at night after a few wines (doesn't always work though).