Tl;dr: Britannia is the ship for people who want the cruise experience while forgetting they're on a ship; beautifully decorated but strangely designed and lacking the service level you'd expect if you've previously sailed with P&O (though I want to believe this is due to specific problems P&O are currently facing which will be resolved in time).
The flight to Barbados and embarkation process were smooth (dare I say enjoyable?!) and my first impression of Britannia on boarding was that she's very elegant, with beautiful decor and well-appointed public spaces. After a long day of travelling plus the inevitable lack of sleep the night before a holiday, we had a quick bite to eat in the buffet and went to bed. The food was superb and (perhaps unfairly) set my dining expectations high for the rest of the cruise.
As this was a repositioning cruise, it was a whislestop tour of Barbados, St Lucia and St Kitts (all beautiful and I would gladly return) before 5 days at sea crossing the Atlantic, a break in the Azores and a final 3 days until Southampton. I chose this itinerary specifically for this because I've always liked sea days and wanted to experience a transatlantic. And this is where the disappiontment set in, due to both the ship and the entertainment programme. The crossing itself was fine - calm seas and good weather for the most part and I would gladly do this again, on a different ship and probably with a different cruise line.
A decent size, with very good storage comprising an open hanging space, a cupboard with a door and shelves, a safe, a large cubboard in the main cabin, a fridge and 2 bedside cabinets with drawers. The bathroom is cosy with ample shelving in the main bathroom and the shower, which has a door. Shower pressure is adequate but nothing special. Cabins include a kettle and tea/coffee which is replenished as used by the cabin steward. You can always ask for extras if you wish, and they'll be only too happy to help. Tip: take some milk from the buffet and put it in your fridge if, like me, you find the sachets of milk are never enough.