When anything is sent from the USA to the UK (or vice versa) it has to have a Customs Declaration on the outside that states the contents and their value. The Customs have the right to open a package in order to inspect it so as to be sure the declaration is correct. It doesn't matter whether it is by FedEx or any other carrier.
If someone visiting the UK were to bring an iPad in to the country they would have to exit International Arrivals by the Red Channel (something to declare) where they would have to pay the VAT and Import Duty.
If they tried to come in through the Green Channel (nothing to declare) that would be illegal. Customs officers have the right, and often do, search a passenger's baggage for undeclared items. It is an offence - subject to imprisonment - to try to bring undeclared items through the Green Channel - it's smuggling.
Passengers are allowed to bring in a small 'personal allowance' of goods duty free - for gifts I think the allowance is £250. Multiple passengers are not allowed to combine their allowances - so two passengers could not combine their allowances to £500 to bring back an iPad.
It really is considered a very serious offence to try to bring goods back into the country and avoid paying VAT and duty - the 'Oh dear, I forgot about that iPad' excuse is not accepted.
I know some people who try to unpack their goods to try to make them appear as though they were bought in the UK and were simply being used overseas and are now being brought back. Customs officers are very adept at spotting this so don't be tempted.
Bottom line - you need to pay Import Duty and VAT if you're bringing goods into the UK from overseas in excess of the Duty Free allowance.