UK iPhone 4S, ATT Go-Phone Sim?

You'd probably be better ordering a red pocket sim off ebay UK. They are an AT&T MVNO. Use them every time I go to USA.

 

Straight Talk, $45 for unlimited everything (2GB data).

 

The paygo sim is easy - they'll give it to you free in store, and you can top up at any time. (They'll also tell you iPhone data doesnt work - but you can add data after, and for it to work you have to modify the apn)

But recently they made changes to paygo - you need to be on $25 monthly plan to be able to add a data option.

 

Depending on current carrier the apn is accessible under Settings -> General -> Network.

Otherwise easy way (what I usually do) is change it by visiting unlockit.co.nz on the phone. You can quickly choose country and carrier and it'll make a profile that gets installed to update apn.

 

With red pocket look at the payg monthly plans.