Or, Buy a SIM Card
I had decided ahead of time to buy a prepaid SIM card when I arrived in London so I would have a British phone number and cheap data, texts, and calls. When buying your SIM card, be sure to have the store's associate give you the SIM card for the iPhone 5, as it is a different size than the SIM cards for other phones. Also, have the associate install the SIM card for you before you leave the store. That way, you'll know that your prepaid amount was loaded onto the SIM card, and you won't have to struggle to open the annoying iPhone SIM-card door with a paperclip back in your hotel room. Returning to the States, I had an extended layover at JFK, and I simply asked an associate at an airport cell-phone store to open my SIM-card door for me—which he did, free of charge.