I have a vodaphone sim card. I have the passport option on the card. You can wait and buy in the UK, sometimes it is free. You can call CS and give them your U S credit card number, but you must give them a UK address. this all of 15 min. to complete. If you want to buy before hand use this company they maild to me here in the US.
www.freedom-mobiles.co.uk/ They are very good and shipping is free.
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Phones Gsm Iphone6+