Firstly make sure you can't get it here, including having a ring around the UK retailers asking if they have it but not listed online or if they can get it
Assuming you have to import have a look for European retailers before going US
Shop around the US retailers for who does post internationally and what they charge, possibly email them to negotiate a different courier
A slower delivery can make a difference
Cost it all up - item price, postage. Then look into import duty
If it is valued below £15 then here is nothing to pay
The gift threshold is £36 (as from 1 jan 2013, previously £40) but that does not apply as you are buying
(Note the value is based on the senders customs declaration but they may also inspect for invoices etc or just not believe the stated value)
If payable then you get a 20% bill from customs, but the courier will deal with you and charge somewhere between £8 & £20 as a fee for handling the customs payment
It is normal practice to either write to you with a customs bill, or a card. You pay and then its delivered or you pay at the local depot and collect
On occasions there have been cases of delivery being made, then he recipent getting a bill in the post later
http://www.hmrc.gov.uk/customs/arriving/arrivingnoneu.htm