Buying an Iphone 4s in Us to use in UK.

You will have to use a plug adapter for the charger. Warranty work will require the phone to be sent back to the US. Make absolutely sure you buy an unlocked iPhone. Any other will be unusable in the UK

Don't forget sales tax will be added to the advertised price, and unless you intend smuggling, 20% VAT and import charges will be payable in the UK. You're not comparing the UK unlocked price with the carrier discounted price in the US that requires a two year contract (which would be useless in the UK) are you?

You would be best advised to keep things simple. Just buy it here.

As stated by the other poster there is no benefit from buying in the USA.

Your phone will not have a UK warranty which will be a pain if you need it fixed.

The price in the USA for an unlocked phone is:

16GB - $649 plus USA Taxes, plus UK VAT and Import Charges that you need to pay at the airport to be legal.

So a 16GB iPhone will be approx $649 + $50 + $140 = $840 at least, which is approx £543

A 32GB iPhone is $749 plus the Taxes etc, and a 64GB iPhone is $849 plus the above.

A UK unlocked iPhone 16GB is £499, 32GB is £599 and 64GB is £699, all taxes included.

So it is no cheaper to buy from the USA nor is it better as you will have no warranty.

As of right now, it's not possible to buy an unlocked 4S at retail in the U.S. So, unless you have a U.S. credit card with a U.S. shipping and billing address to order it from apple.com, you can't buy one anyway.