London To Venice

Take the train get one those couchettes if they are available as single (assuming you are traveling solo) that sounds like fun.
Maybe not:
https://www.seat61.com/thello-train-from-paris-to-italy.htm#1-, 2- or 3-bed sleepers

Eurostar to Paris in early morning. Two days (one night) in Paris then the night train to Venice. Or 2 days and 2 nights in Paris and fly to Venice in the morning of your check in day in Venice. Either way would work for a quick Paris visit.

If you have the money, this looks pretty nice:

https://www.belmond.com/venice-simplon-orient-express/

I saw this train in the station in Venice one time and it looks like they pull out all the stops...

Did opposite - night train from Venice to Paris, then London. Night train a joke. Hot, A/C out, continually stopping and not informing us as to why, breakfast a bigger joke, never again. If you decide not to stop in Paris - take a plane London to Venice. More time in Venice.

If you want to stop in Paris, take the train to Paris then fly to Venice.

As stated earlier, not everyone sleeps well on trains. So, instead of about 14 hours on a train from Paris to Venice, take a flight and get a good night sleep in a hotel. This way you get to hit the ground running in Venice rather than dragging yourself around due to poor sleep.

Or.....skip Paris and stop two nights along the way via train. Something like London-Basel-Innsbruck--Venice. Nice scenery especially once in Switzerland. (And especially after Zurich.) You could spend the mornings sightseeing and take an afternoon train to your next destination.

Day 1. Eurostar from London to Paris
Day 2. TGV from Paris to Lausanne
Day 3. Train to Milan for lunch, dinner in Venice

On your Venice thread, I recommended flying. It makes no sense to me to give Paris such a short visit when you're being more generous with other cities. However, a small town near either London or Venice can be visited more thoroughly with two nights. The night train does not get great reviews and flying will also be much cheaper.

I read on the France thread that some posters think there are bonus points or train travel!

I go with the bonus points! I love the Eurostar. I love Paris. We had a wonderful time on the overnight train from Paris to Venice. We had a recently-widowed French lady doing a walking your in the mountains outside Turin and an American expat now living in Venice. We had a great sleep, met nice women and woke up raring to go in Venice. Actually, woke up in time to experience the arrival into Santa Lucia. A must see.