O2 the exclusive carrier in the UK have sold out of all iPhone 3GS models both online and in store.

From O2′s website:

iPhone 3GS is currently out of stock:
Due to the phenomenal demand for the new iPhone 3GS, we’ve temporarily run out of stock online, over the phone and in our retail stores.

We expect additional stock of the new iPhone 3G S to be available online, over the phone and in our retail stores at the end of this week.

Carphonewarehouse, O2′s partner in the UK has stock of the 32GB iPhone 3GS available for purchase but are not currently listing the 16GB version as available