An Expat's Guide to Visiting London: Mobiles working Mobiles chirping

Getting a mobile phone working in the UK is pretty easy.  Here's what you need to know.

First, an iPhone or other smartphone with GPS and maps will be a huge benefit.  In fact, you will probably want GPS and data more than you will want phone minutes and texts. 

NOTE:  Verizon phones are on a different type of network (CDMA) and won't work in the UK.  It needs to be a GSM phone (AT&T, T-Mobile). Wikipedia more info on US cellular providers and what network they use.

Step 1:  Before you get here
Make sure the phone is either unlocked or your provider allows for international SIM cards.

AT&T has gotten a lot friendlier about unlocking out-of-contract iPhones, so if you have an old iphone that is not under contract, it should be unlockable.  Check AT&T's website for instructions.  It may require a phone call, but hopefully just a web form. 

You can test your unlock at home by switching someone else's SIM who's on a different (GSM) carrier and see if the phone works.

Some new plans allow international SIM cards as long as you keep paying the bill back home.  If your current phone's plan allows this, then you should be good.  Again, double-check that both your provider and your phone allow this before traveling so you don't get stuck.

Step 2: After you get here
Once you get here (the easy part) go to any phone shop (EE, O2, Vodafone) and tell them you need a pay as you go SIM card for your phone.
£10-15 should get you more than enough minutes & data for the trip.

NOTE:  There are SIM vending machines at the airport.  Skip these if at all possible and wait until you can go to a proper shop.  There is a lot less risk of getting a bad deal from a shop.

It is so much easier here than in the USA where you typically have to get a single-month contract and pay a lot more money.