Been to an O2 store, spoke with a head honcho. The BTOpenzone link is confirmed, they're not planning pre-order (but Carphone Warehouse is - and it'll be announced soon). CW dones't know there pricing, but expects same as O2 - O2 knows it's pricing.
As of today/tomorrow pretty much, O2 shops will be pushing Simplicity, hard. Then from this, the switchers just buy the iPhone at store for example, activate, and then you get a temporary number for 1-2 days usually, then port your number over. A very simple process, and O2 with the publicity from the 3G iPhone launch filtering into national news and coffeeshops and water cooler conversations will then be able to snag a lot more people.
Hope that's enough info for you