Day tour from Portsmouth, UK to Mont Saint Michel

Going from Portsmouth (or even London) my choice would always be the overnight ferry crossings to either St Malo or Ouistreham (the port of Caen).
Caen is the shorter and more frequent crossing, and probably the more obvious.
Both routes are very much foot passenger as well as car passenger.

And yes I do read that it is October.

I'm not looking at cost as that isn't coming into the equation for me, but the ferry should come in well cheaper than a Eurostar/SNCF alternative via Paris.
This is all about convenience to me. Ideally I'd be adding on a day beforehand for St Malo.

There just isn't a sensible flight option - those from Southampton airport have waxed and waned over the years.

My only thought is that it will be almost impossible as a day trip.
The Portsmouth to Caen Ferry is 6 hours each way, Cherbourg and St Malo are even longer.

As Lin C says, it seems impossible as a day trip. There just isn't enough time. You could do this perhaps as an overnighter, but the logistics sound difficult and complicated. Plus, October weather could interfere with sailings.
If you do find a way, let the forum know.

We all have places that have a strong pull to us as individuals. Perhaps Mont Saint Michel is one of those places for you. I agree that this makes little sense for almost everyone, unless you have a special yearning for this location that you can’t ignore.

If it was just an idea, I’d encourage you to drop it. If it is a dream, I’d encourage you to give it the time a dream deserves and add at least one overnight in France.

MSM also doesn't have particularly good public transportation from a distance. Many posters here are staying in Bayeux, and they take the daytrip (from France!) run by the Churchill Hotel in Bayeux, which accepts non-guests for the shuttle bus. You can't take the train all the way to MSM, you have to switch to a bus for the last few miles. We had a rental car for a longer trip, and stopped there for a few hours one morning, which worked well for us. Settled for a pre-made sandwich while walking down the "medieval street".

https://community.ricksteves.com/travel-forum/france/travel-to-mount-st-michel

In addition to the challenges already mentioned, there's the fact that cross-Channel ferries are sometimes canceled. I guess that doesn't happen particularly often, but two of my five Channel ferry sailings have been canceled. Both trips fell between May and September.

Thank you everyone for your great suggestions! I am still at the early part of planning stage, and definitely I need to give it more thought to this trip.
Thank you!

Thanks for the tips!
We've decided not to go there this time...not enough time.