Would I have to declare an iPhone at customs?

I'm heading out to the US. After some research, I can buy an american iPhone 5S (unlocked contract free) for £385 (currency converted), a saving of around £165 from the UK model (I've asked around and it works on the UK carrier I'm on). Anyway, I know for items under £390 you need to declare at customs at the airport. The UK price of this is over £390, but the american one is not (where I'm buying it, it's under £390). Would I need to declare this to customs at the airport? If so, then there's no point in me buying it because when they add all the charges it won't be worth it.

Thanks.

Added (1). I'm going from USA > UK, so it's UK customs!

If you declare, you have to pay import duty plus VAT. If you get caught not declaring it, you will pay import duty plus VAT plus extra duty plus extra VAT fines plus smuggling fines.

Buy one in Britain, or better, buy an Android smartphone - they are all better value for money than the iPhone.

I'll guarantee you you won't get the phone at that price. I'm a regular visitor to the states and can say quite categorically that Apple products are no cheaper over there. Some retailers in the states advertise technology at massive discounted rates to get you through the door and sell you something else.