You basically get to import a slightly higher value of goods (something like $70 instead of $30) without getting charged in UK. They also charge items declared as Used differently here. I've heard enough about customs in Finland from annanas, they are really strict there. It's a bit more hit and miss here as I received two pairs of jeans from Revolve yesterday, declared at full value and didn't have to pay anything.
But, like DQ said, this only happens to items posted by normal mail, anything posted by a courier like FedEx will get charged depending on the declared value, there's no getting around it.