it should be okay without the reciept, apple are usually pretty good about replacing faulty units, i think they can check the purchase date by the serial number or imei or something like that. in regards to the ipads in the brown box there is a chance they will be refurbs, but dont be concerned as they will be in as good condition as new, they are not the same as a second hand ipads, the likely hood is that you wont be able to tell the difference between a refurb and a new boxed ipad.
in regards to light leakage be aware there is a chance you will recieve an ipad with the same problem, i have swaped out 6 since the uk release and all have suffered from the issue
regards,
and good luck