We are also on this cruise - there is a link on the P & O website - if you enter your booking reference when asked for 'voucher code' (or something similar) it is free.
Southampton parking can be with CPS there website is www.candps.co.uk
BCP parking there website is www.parkbcp.co.uk
Airparks also do southampton cruise parking in the airport drop down box there website is www.airparks.co.uk
hope this is of some use.