I am almost certain there will be a Simplicity type option. This is what happened in the States.
If you bought from an AT&T store (read: O2 or Carphone Warehouse), then you generally did the credit check there and then before you bought it.
If you bought from an Apple store, you just pick up the phone and activate via iTunes. Here, a credit check is conducted. If you passed the check, then all is fine and you choose one of the 3 plans. If you failed the check, then a new plan becomes apparent:
http://farm2.static.flickr.com/1077/902403648_d62856886a_o.png
http://farm2.static.flickr.com/1113/902404998_592710f3dd_o.png
So I believe there will be a prepay option, however you will not just be able to walk into a store and purchase it that way...and O2 certainly do not want to promote this as they want you on a nice long 18 month contract. The bizarre thing here is that failing the credit check works to your advantage as you are not locked in and can change your plan from month to month.
But yeah, if Apple are selling the phone without a contract at their stores, then there MUST be a prepaid option like above.... otherwise you will get a hell of a lot of people buying it, attempting to activate at home and not being able to use it. Which makes no sense.