The only problem with Paris is that the ships dock a couple of hours drive away, so there is not as much time to see paris as you would like. What I would suggest is that you maybe fly to Paris CDG instead of London & stay a night or two to see it properly.
From Paris CDG you can get a direct flight to Southampton for the cruise - which would be a lot easier than London, or from the centre of Pairs (Gare Du Nord) you can take "Le Shuttle" into London Waterloo, which is the correct station for trains to Southampton.
From Southampton you have Princess, Cunard, P&O, RCI, as well as some of the budget cruise lines.
An alternative to Southampton is Dover which has a lot less lines, and would mean a ferry crossing from Calais in France, but thought I'd mention it.
Hope this give you some ideas, if you have any questions, feel free to send us a private message & we'll do our best to help.
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