There's a lot of cruising snobbery with the judging different "classes" of passengers so here's the deal, P&O are clever enough to cater for a broad market, love it or pay more and go Seabourn!! The joy of a big ship is that you can avoid the areas which aren't to your liking and if you feel that way inclined the "types" you might want to avoid are away from the areas you probably enjoy.
Yes, the ship decor is rather dark and my first reaction was to pine for the bling of Ventura but sultry Britannia grows on you. The public areas are cleverly designed with many nooks and crannies so you can be in the heart of the action or tuck yourself away as you choose.
The Glass House is, as ever, excellent. The Crow's nest bar is back at the front of the ship and wonderful for pre dinner drinks. Recommend sitting away from the piano as it can be very loud. The atrium is very spacious with plenty of seating to people watch with a post dinner drink/coffee. Drinks service was excellent everywhere. If we were waiting we only had to indicate to someone and they were there. Crystal room was surprisingly pleasant late evening with lots of people taking to the floor. Didn't frequent Brodies or the casino but it was popular and pretty packed each time we walked through.
Mini suite was spacious with clean lines and kept spotlessly clean. Small gripe was the electricity; sockets don't accommodate standard electric toothbrushes so adaptor is needed and hairdryer didn't stretch to mirror, hardly life changing but mildly irritating. Balcony OK to sit outside but forget using it for sunbathing, it's far too small.