Typically the Southampton cruises don't board people at different ports, unlike the Mediterranean. That means they don't let you back off once you're on board.
There's not too much to see in Southampton itself, at least in proximity to the cruise terminal. You're best taking a transfer tour that will take you to Stonehenge and Salisbury on the way down (try for Avebury too if you can - far more enjoyable than Stonehenge).
On cabin, would echo the recommendation to book one with cabins above and below, and aim for midships. Beyond that, they're all the same, as long as you avoid one next to a service area (white space on the deckplans).