Want to know how to buy iphone in usa to gift someone from uk

Unless you have a valid US payment method, you won't be able to order an iPhone from the UK in the Apple US online store. The best option if you want to gift someone an iPhone is to send them the money or send the money to someone else you know and ask them to buy the phone and you can reimburse them.

Here is a link to the Apple.com US website --> Apple

You can't as already explained. Asking again won't change the answer.