We're going on Iona (G312), this will be our 3rd time on board her. Definitely wouldn't be concerned about getting into shows, the theatre is large enough that if you are a little flexible about when & where you'll get to see everything you want to. Same goes for the restaurants, but if there's something you particularly want to try (e.g. Epicurean) then you can book now before you board.
Even if the P&O excursions offered are full, you should be able to find independent operators doing the same excursion, these ports are used to having large ships and from the research we've done it appears they cater for them very well. We prefer to make our own way around, all the ports look very walkable.
Weather will be what it will be, take layers, you could need an umbrella, or a parasol.
Same situation with the ports, nothing is guaranteed, but in that regard Britannia may offer more certainty. Keep your fingers crossed & hope for the best, if you can't get off the ship then Iona (in our opinion) is by far the better place to be stuck in.
At these prices, treat it as a sampler, you can always do it again. Hope you enjoy the cruise, whatever you decide.