Biggest benefit is avoiding airports by choosing cruises that start in UK ports!
They may have huge numbers of people on them, but they're huge so you can easily spread out, usually multiple bars/lounges, multiple restaurants, several areas on deck, etc.
Breakfast buffet time can be manic, but you can easily avoid the busiest times by getting up a bit earlier, having breakfast delivered to your room, using a different restaurant for breakfast, or staying in bed and waiting till the rush has passed. Breakfast buffet is usually served between around 6am to 11am, so lots of opportunity to go at a less busy time.
Waking up in a different town/city/country every morning is amazing. You have choice of a "proper" day trip/tour, or just leaving the ship to walk around the town, or staying on board to enjoy the almost deserted ship. Lots of different options for leaving the ship - the cruise firm put on maybe a dozen different excursions on each shore day, or you can usually get shuttle buses from the dockside to the town/city centre, or get a hop on/hop off bus tour of the city, or even a taxi, if it's too far to walk.
Usually excellent entertainment too, either shows in the theatre, or cabaret singer in a bar, or organised sports/games by the entertainment team, usually a disco/club.
Basically, if you can think of a posh hotel right in the middle of a vibrant town/city centre, then that's what a cruise is.