I'm writing this review as my others in sections. Sailing from Southampton UK.
As with all of my other P&O cruises, P&O have got this to a fine art. I had to be out of my digs at 1200 and I arrived at Southampton Docks around 1300. My car was taken to a long-term car park (provided free by P&O) and I went into the main check-in hall. This hall is huge with literally hundreds of people waiting. There were dozens of P&O check-in people on one side and we all had a coloured card with letter for our times. My check-in time was 1530 so I had a while to wait. I eventually checked in at 1500 and was onboard by 1530. I will say that there wasn't any refreshments available in the hall so if you're a bit hungry/thirsty, you'd have to wait till you're onboard or bring something with you.