I’ve done a few cruises, and enjoyed them all.
Royal Caribbean is good, but expensive, and drinks pricey too. Food was quite American, but buffet choices were superb. Fantastic shows, eg We will Rock You, Grease, plus very impressive ice skating shows.
Virgin was great for the food options, but the ship I was on, only had one small outdoor pool, and I like a few pools to choose from. It made the hot tubs very busy, and some people hog them unfortunately.
P&O is very British, but I quite like that, eg, roast dinners, sponge and custard etc. Drinks a reasonable price. Great entertainment too. I’m going on Britannia in 2023, and can’t wait.
My DD has been on MSC, and enjoyed it, but didn’t rate the entertainment, unless opera is your thing. My Dsis loves MSC, but they do Yacht Club, where you get a butler and your own restaurant, pool etc (mega bucks for that luxury though).
My next cruise will be on Celebrity, because everyone raves about them! They have a new ship sailing out of Southampton next summer I think.
Personally I love sailing from Southampton, because I live fairly close, and getting guaranteed good weather doesn’t interest me particularly. Plus none of the dreaded airport waiting around stuff.
You’ll have a ball no matter which cruise line you choose.