The $ -> £ may happen or it may not, depends on the site.  Either way you should expect a bill for 20% VAT (same as on anything bought locally) as well as a fee from Royal Mail or the courier for them paying the tax man for you, usually you have to pay this before they will deliver so it adds a delay as well as expense.  Sometimes you get lazy people working in customs and things get through but more often than not I've had to pay.