According to the 2008 P&O brochure, weddings should be possible on all their ships in 2008. But, you should contact them in advance to ensure that this will be available on the World Cruise.
We're getting married on the oriana in december. details of weddings on board can be found on the p&o website (http://www.pocruises.com/pocruising/learnmore-celebration-cruises.aspx) If that hyperlink doesn't work, look under the "cruised before" tab.
They seem to cater for whatever size/style you want, but you do need to book 3 months in advance. the web page brochure explains what docs are required. Best wishes for your weddin.