I would leave calling o2 for now. I am in a similar position to you, I am actually from the North but am moving away to Canada on June 30th so will be in a hurry to unlock the iPhone 4 before I leave. I am therefore also looking for a PAYG handset. If you call o2 now, they won't be able to tell you what you need to know. I/you should wait until they announce pricing and whether a PAYG option will even be available on June 24th -lots of people are assuming that it will be available at launch like last year, but demand will be much higher this time around so options might be limited. So it is possible that we will be disappointed, as the initial release could be for contract-only handsets. I have also never seen so many people talk about buying the non-contract phones before, I think people are finally seeing the benefits of buying the iPhone upfront as they hold their value so well.
Hope this info helps.