Oh yes we can
@King Edward I
You missed the point.
The iPhone is the commodity and O2 are the group that have exclusive control over it's sale in the UK.
If I want to own an iPhone in the UK without signing an O2 contract (monthly or PAYG), I would have to import one myself from territories like Italy or Hong Kong where it's available unlocked & contract free.
Your comparison with Dell is invalid as they manufacture hardware which isn't unique and use an OS which also isn't unique. We can buy this hardware / software combination from any number of vendors which makes the PC non-exclusive (not to mention the fact I can walk into my local branch of PC World and buy a Dell PC).
An iPhone is unique, as even though other smart phones (with very similar features) are readily available from different vendors, only one company (Apple) manufactures the specific iPhone hardware / software combination (making it an commodity) and only one UK telco (O2) controls it's sale (giving it exclusive control).
That, to all intents and purposes, is a monopoly.