Don't know exactly about the UK but here or in Eu you get a call from customs or your freighter with the bill for inport and VAT (GST).
After they received payment they release the goods.
There is normally a website from customs and taxations where you can do a search on the import tax for that particular product/goods.
Shouldn't be to hard to find I suppose.
Anyhow that is how it works in General, I'm sure one of your country man will be able to clarify.

When I've had stuff sent from the States I have always received a letter/postcard saying that my item is ready to be picked up from a Post Office central sorting office and that there is duty to pay. Then you have to go and pay it before they will release the goods. It's always a lot of cash, probably it is about 15% to 20% but I'm always so eager to get the stuff that I just pay it and swallow my frustration about it.

I think that the payment you have to make is the VAT.

For Belgium
Value of the goods is the total amount paid (price of the goods + shipping)

Value below EUR 22 -> no charges
Between EUR 22 and EUR 150 -> VAT (21%) + customs handling charge (when the package is delivered by Bpost: EUR 12.50)
Above EUR 150 -> VAT + customs handling charge + import tax

The amount of import tax that you have to pay depends on the type of goods. There is a list available on the Belgian Customs website.

I guess it will be similar for the UK? http://www.hmrc.gov.uk/customs/tax-and-duty.htm

EDIT: http://www.dutycalculator.com

Expect to pay 20% plus a handling charge (about £12), unless you buy from a private seller and they mark as gift with a low value (but you can still get stung).

I have had a £200 used radiator ordered and paid nothing, but also had a £22 item of clothing arrive and paid £12

Can be hit and miss, but "gifts" from non business sellers is the way forward, sellers mostly will not mark as gift as they can get hefty fines if caught.

sometimes it can still be cheaper to buy from US even with the tax hit.

you CAN ask whoever is shipping to list as a warranty replacement and pay no duty or VAT.
This definately 100% works!!
It isn't right of course and we shouldnt condone such behaviour