There are two choices. She could pay AT&T for international data, but since she'll be there for a two and a half weeks, it would be pretty expensive. Leo would probably buy a SIM card when she gets to the UK from Vodafone or O2 that she can put into her iPhone which would give her a UK phone number and unlimited data. This would be much more affordable than paying AT&T for international data. She could do the same thing when she goes to Sweden.