make sure you need one first, then get it sent via UPS (they ship it direct to you and sort customs as well) do not use USPS (US postal service) as they are slow and ship it to uk customs only, then parcelforce will charge extra to move it from customs to your place.
if you buy the one for under $20/£10 you probably won't pay any import duty and worst case it will be £2 (y)