I'm in the same boat - I was looking at the Orange PAYG "Racoon" option as it is the only one that reads as if it doesn't have a minimum monthly top up in the terms and conditions. All the others ask for at least £10 - I know most folks will think that is nothing to spend but I really can't see me using even that much on calls/texts as I want it many for the web/mail and 90 odd percent of the time I will have Wi-Fi available anyway.
As sourcing the phone from Orange will likely mean it is locked I was considering getting it from Apple direct but it isn't clear if you can get the "Racoon" PAYG micro SIM separately. Alternatively I could accept it being locked initially if there was a cheap official way to unlock it. I know with O2 they charged £15 to do this on a PAYG unit with a short wait before it happened.