the dress code is here
http://www.pocruises.com/Help/Dining/
I don't know about P&O, but on other ships I've sailed on, they only apply to the main restaurants and if you don't want to get gussied up in a suit and tie you can head for a more casual restaurant. Without knowing which ship you're on, I can't say howmany restaurants there'll be, but maybe half a dozen.
Personally, I travel for the destinations rather than for the actual travel, so I always go on excursions when we get to a port. Ships run their own, which guarantee you will be back in time to sail; but you can often arrange one for half the price, or just stroll around town, and still be back in plenty of time.
What you've paid probably won't include drinks (which can be pricy) and may not include tips either - better check this, as tips are all but compulsory and can be quite a chunk of your budget; but British ships often include them in the price.
You can look up your cruise here and see what otheres have thought of it in the past:
http://www.cruise.co....&reviewdestinationid=
cruisecritic is another website to look at.
Generally.... you're going to have a lovely time. P&O are a good line.