Choose the option 'Pay for your device in full' found lower down the link & the 1Gb option to keep it to the minimum in case you get charged for the 1st month.
Like all O2 Refresh deals, call & cancel the Airtime when you receive the phone & the phone is yours to keep for just £326.
This generates a credit check, so not a good idea if you'll be applying for a mortgage, credit card, etc in the next few months.
Make sure they cancel the Airtime properly & ask them to send you an email confirmation so you have it in writing in case of future disputes.