I'm travelling to London this week for a week and then will be in Paris for a week. I have an unlocked iPhone and am planning on getting a Pay As You Go SIM card from Three for £15. Am I able to add the Feel At Home option with Three to use this SIM card while I am in Paris or do I need to find a different SIM when I go to Paris? If I need a different one, what do people recommend?

Thanks!