I can only stress that as a fellow iPhone user that I advise strongly against their network. Orange was great but when EE hit I've had the most miserable year of them because they amalgamated and removed too many masts from the amalgamated networks to save money, hence, all over London and out is nightmare. Their customer service is a joke because not once have they called back when they said they would.
Perfect example. Local restaurant last night. No service. Two other people on Vodafone and O2 full signal, watching videos, texting, tweeting etc. And Kevin Bacon ramming down my throat every night on telly that EE are the 'biggest and the best'. I promise you they're not.
I've no experience of 'Three' but they can't possibly be as bad.