Marie is traveling to Great Britain and is worried about data charges on her iPhone 6. Leo says that Marie can go into her phone's settings and turn off Data Roaming. That's where the real data costs will rear it's ugly head. Then she can just depend on local Wi-Fi. She can also download the maps and keep them in the phone so she won't have to worry about getting lost with no data.
Another option is to buy an international data roaming plan, which includes 250MB for around $100. There's another option still, though. Leo says he was in Britain a few months ago with his iPhone and the nice thing about the iPhone 6 is that it has a SIM card. She can pop out her SIM and buy a local SIM in Great Britain. That means she'd have a local number and data plan. She may need to ask AT&T to unlock it in order to do that, though. But if she's had her phone for at least six months, she shouldn't have a problem.
Leo recommends checking out prepaidwithdata.wikia.com for details on what cards to buy locally.
Lastly, she can get a MiFi card which will create a Wi-Fi hotspot in her pocket, and she can put the local SIM in that.