You'd be forgiven if you thought buying the iPhone 5 would allow you to use any 4G network you like as and when they become available. Unfortunately this isn’t the case and the options are limited. See also: iPhone 5 review
The problem lies in the fact that EE has been allowed to roll out a 4G network ahead of the competition. EE will launch 4G for consumers in the next few weeks and will initially be the only mobile operator to give iPhone 5 users 4G mobile broadband speeds.
EE's network uses the 1800MHz spectrum which is supported by the iPhone 5. Although O2 and Vodafone will be bidding for 4G spectrum in Ofcom's upcoming auction, the 800MHz and 2.6GHz spectrums do not match the iPhone 5's 4G specification.
Orange and T-Mobile customers won't have the option for 4G either. However, since they are owned by EE it will be easy to move over to the new operator.
Three is the exception here since EE sold parts of the 1800MHz spectrum to Three which it can use for 4G services. We've been in contact with Three to find out when it will launch 4G and are waiting to hear back.
In summary your options with the iPhone 5 are to go with EE or Three for 4G when the networks are live or use the iPhone 5 on existing 3G networks from O2 and Vodafone. If you want an iPhone with 4G on O2 or Vodafone then you'll probably have to wait for the next generation of iPhone.