TB: I've done about 5 or 6 so far, either on Royal Carribbean or MSC. I was similar to you, in that I thought it would be a terrible idea but my partner and family have always done it so persuaded me to try it. A few observations:
The boats never feel as busy as the numbers on board would make you think they might be. The only exception to this is Round the swimming pool(s). For engineering reasons these have to be small and inevitably get crowded.
Don't sail out of the UK for anything less than 7 days. If you do you're likely to get a lot of stag and hen parties, with all the bullsh!t that entails.
Having a balcony cabin is a big bonus as it means you can always just sit and watch the world go by with no one around.
I would always book through an agent - I've personally used vacations2go (denominated in USD but have thousands of cruises listed in a very no nonsense website) and cruise.co.uk. it's usually worth using these as you can typically get a good chunk of discount.
Finally, all cruise lines have a different ethos and target market they cater to. So think hard about what type of holiday you want as picking the wrong line will likely lead to you having a poor experience. We've booked the Carribbean for Easter 2025 with MSC, 2 adults and 2 children for 10 days and it's £3,500 'all inclusive' which for the Easter holidays is cheaper than center parks. That said, we picked MSC because it's family focused, tons for the kids to do and lots of shows. But it will be busy and it will be full of kids. Although Rangie hasn't said, unless he's going with grand children I'd be surprised if he's booked the same cruise line as (in his position) I wouldn't want to be surrounded by children on my relaxing beach holiday.
Lastly, on your sailing from the UK point, I found out this week that my partner has booked a 7night cruise from Southampton in may for my 40th. That route is Southampton -> at sea -> Bordeaux -> Bilbao -> la coruna -> at sea -> le harve -> Southampton. We will probably stay on board at le harve, but the other stops will doubtless have something interesting to offer.