Some Apple customers bringing a white 16GB iPhone 4 to Apple Store Genius Bars for repair have been lucky enough to walk away with an iPhone 4S due to shortages of the entry-level model. 

Apple notified its stores that while the inventory of while iPhone 4 16GB is constrained they should upgrade customers seeking a replacement phone. 

Unfortunately it appears that the UK Apple Stores are not experiencing the same constraints. 

9to5 Mac's sources were based in the United States and Canada