@ Robert Hill
"Unlocked iPhones available in other countries are very expensive - and IF the Apple stores in the UK sell unlocked iPhones I would expect them to be very expensive (>£600) as well"
Not strictly so, a "locked" (to o2) PAYG phone from Apple's UK store is what £540 for a 32GB?
The same model (completely "unlocked", by Apple themselves no less) from the Hong Kong Apple store is about £490 so we actually pay more in this country to have them locked to o2.... (obviously exchange rate fluctuations, but basically I think Apple have aimed for a consistent price between countries for the "PAYG" models they supply themselves)
There is no reason why the PAYG phones supplied by Apple in the UK can't be supplied without operator locks (when the o2 "exclusive" expires at the end of next month I mean)
The "Unlocked ones" you mention are I suspect the ones that are "grey" imports from places such as HK and Italy - I saw one at Dubai Airport yesterday - >$900. I think play.com et al also import them from places that have them unlocked by default and slap a hefty mark-up on them.
My point is, if Apple apply the same logic to the UK PAYG phones they supply, there is no need for them to be more expensive then they already are.