Apple, O2 and Carphone Warehouse all carry the 16GB model, which retails at £329.
The spokesperson added that the new phone will also be supported by Orange in France and T-Mobile in Germany, although a release date for those countries was not given.
Apple released the new iPhone on 5 February along with a 32GB version of the iPod Touch, which is currently available worldwide.
The announcement marks the first time that Apple has made two versions of the iPhone available for UK users. The company discontinued the 4GB iPhone in September, two months before the device was launched in the UK.