Seeing as Carphone Warehouse is going to have pre reg, O2 could easily do this, via there email system - they already have a preregister interest, i'd imagine this will post something on .. Friday it matt your timings are right (I haven't spoken to any O2 rep for a few days, and things move fast).
If you do pre-register, you should be ok, and you'll have a handset reserved, but just have to wait in line to get it unlocked and registered.
Seeing as they're will be a queue at some O2 stores (but more likely at the main Apple stores moreso than local O2 shops), it may be quicker to get it just after all of those have been served. Lunchtime will be a busy period for htose not able to get in before work. Not necessarily too bad, as people may get up early for the 7.02am opening, giving enough time to buy, and commute. Or just do it after work.
O2 stores didn't run out of stock in the UK opening. They'll be well stocked. Afaik, the only stock problems have been last month or so, which was for completely different reasons. good find on the date

Hopefully some more people can get confirmations about CW and O2's pre reg plans for it seems Friday.