I recently asked members about custom charges when buying from the US, and after talking to a couple of people over the phone (in both the UK and US) I think it is just a lottery as to whether you get charged or not.
But I did find out that if you want your order delivered fast from the US, then you should ask the company you are buying the item/s from to get you a quote from USPS (not UPS who are a lot more expensive) using their Global Express Mail which takes 3 to 5 days from when they pick it up. I was told that you also have more chance of getting it through customs without any charges using them. I had a parcel with items worth $360 within it and I never got charged a penny, although I suspect that this might have been pot luck.
It will cost you a bit extra using their Global Express mail ($59 for the above parcel) but if you want your order quick then this is the way to do it.
They will tell you that the only thing that may hold your order up is the customs here in the UK while they contact you to tell you how much you owe them. If this happens then you may not receive it within the 3 to 5 day period, but even then you are only talking about a few days more.
Cheers
Mike