Anything you buy above £18 will be liable for VAT and import duties when entering the uK.
You will also be charged an standard £8 handling charge from the post office, but depends on who they use for international postage, if the use fed ex then it will be dealt direct from them, any other mail provider tends to use the post office when it land to the uk.
Bear in mind that anything above the threshold will be liable for VAT (20%) and import taxes (8%) plus handling fees and the original postage charges.
In my opinion, if you base the $ as advertised! that is what you will usually end up paying in £ when it enters the uk.
In short, advertised at $10 then you will end up paying £10.
Hope this helps.