Firstly, we have cruised with P & O before on the Oceana and Ventura and had high hopes for the Britannia - how disappointed we were.
We booked a family cabin (F629) which had two pull down bunk beds. When the beds and ladders were down there was no room to get from one end of the cabin to the other, you had to climb over the lower bunks to get anywhere. The design was awful and all four of us regularly hit our heads. There was also a lack of storage space for four people - we had clothes under the dressing table and the chair because there was no where else to put them!
The ship itself was decorated well, but that's about it. There was no atmosphere in the Atrium and only a few other venues to choose from unless you were prepared to pay extra. The shows in the Headliners theatre were excellent, we generally went to the 8.30pm showing and always found seats but had to get there at least half an hour early by which time most of the seats in the middle section were taken. The live lounge had some good entertainment but we missed a few shows because we didn't realise soon enough that to get a seat you need to be there over half an hour early!