Haha Abbey I guessed you meant first cruise by your post! Cruising is wonderful and one of my favourite things is formal night which P&O do better than any other line (most are dropping it sadly). Don’t be worried about feeling sick, claustrophobic (usual fears) as it’s very unlikely.
Appropriate clothes during the day are casual and evening smart casual. People dress up on formal night (tuxes, cocktail dresses, some long frocks but not full ball gowns). I usually shower when I get back from a day in port, then change into evening wear. I love getting dressed up.
Britannia is a beautiful ship - like a top London hotel. My friend who went at Christmas (I went 18m ago) has just said the food & service had massively improved so that’s good news.
We never purchase shore excursions in Europe- only in the Caribbean where prices are more reasonable. You can DIY. Where are you going? There will be meet up groups on ship advertised in the daily paper- single, friends of Dorothy (LGBT), etc. I haven’t traveled alone, but I’m sure it wouldn’t be too difficult to meet like minded souls also travelling solo.
One of the best lines for solo travel (for future) is Norwegian as they have solo cabins, a dedicated lounge for singles, organised events, people rave about it & cabins sell fast!
Cruising really is for everyone though. I’ve seen all ages, groups, ethnicities, social /economic groups.... as I say less diversity on P&O but still a great range of ages. On our last cruise we hung out with a fun couple in their 60s, we’re in our 30s, just friendly people wanting to have fun together. Lots of people mixing which is really nice. Cruisers are not the stereotype people think of. We’ve seen top BBC journalists, Olympians (on holiday, not doing talks). There are classical music concerts, jazz, musicals, family events, silly competitions poolside...etc. Really there is something for everyone and if you don’t like something going on you can go to a different part of the ship.