An iPad purchased in the USA will be supplied with a power adapter fitted with a US plug, for connecting to a 120V mains supply (at 60Hz). You won't be able to use it with the 230V (50Hz) domestic power supply in the UK. So you'd need to buy a UK power adapter.
Since the iPad can function as a media player, it's likely that UK customs officials will charge Import Tax at 13.9%. Then there's 17.5% VAT to be added onto the total of the cost price and the Import Tax. So (based upon a retail price of $499 = £345), HMRC will charge you approximately £117 to import your iPad.
So, even before you consider whether you can actually connect a US iPad to the internet in the UK (not to mention the inconvenience of having things like a built-in dictionary which uses US spellings), you could well end up spending far more than if you purchased your iPad in the UK.
Chris