You can't use the 02 site if you are buying it from cpw as i asked about this because i didn't understand why you needed your Pac code and the time of ordering but cpw have there own rules.Yes, there's even an online form to do it.. but it might take a few days to transfer the number.
http://www.o2.co.uk/keepyournumber
''To transfer your number when you sign up to a new contract online, phone us on 0870 087 0870 before placing your order to speak to a member of our team because were currently unable to process this online''