Hard to say overthehill. Depends how you spend your time on shore really. P&O on board costs are all in pounds sterling, charged to a credit card at the end. So anything you spend on drinks, in the on board shops, for speciality restaurants and yes, for automatic tipping. Passengers can ask at reception for the tipping amount to be reduced or removed if service is felt to be poor or it is preferred to tip individually in the traditional way. There have been many strong feelings expressed about this in the past - I'm not saying one way or the other is 'right', just making you aware of the alternatives.
You can ask at reception for an account at any time to keep track. If you book trips in advance they are paid for in Sterling in advance. You can use a credit card for this without a credit card fee. Some trips will include a drink and snack, some won't. If you plan to go ashore independently you are likely to need more local currency than if you do a trip. Dont forget though that the ship will always have food available at whatever time you return.
Once the information is available (on line, you will have access to your booking via a booking code) about trips you can look and see prices and make a plan for each port. Unless there is a very short walk from the berth to the port gates and nearest town P&O will provide a shuttle bus, usually into the nearest town. Depending on your booking these are free of can carry a small charge which is added to your on board account
If you booked directly with P&O your cruise consultant will answer most queries, otherwise it's worth a chat with your travel agent.
Don't worry at all, there is loads of information out there. Cruise Critic has a chat forum and is a mine of information. The P&O site has an 'already booked' section with all sorts of answers to popular queries.
Enjoy your holiday celebration, really hope you have a wonderful experience.