Late in the year is when the weather in the Channel is worst.
It might be a mill pond when you go, it might not. It all depends on what awful weather is sent across the North Atlantic from the United States 3 days earlier.
There are two routes by ferry to consider, the long crossing from Portsmouth to Caen then train from there (the same train from Paris as you would get connecting with the Eurostar, just almost at its end) and the short crossing from Dover to Calais. That crossing is only around 90 minutes in good weather but about 3 hours when all is considered. You have to take the train from London to Dover and walk or get a taxi to the ferry terminal. Foot passengers are only allowed on the P&O boats from Dover and have to hang around quite a while until they are taken on the boat. Similar at the other end, and the Calais train station is quite a long way from the port. Then train to Paris and cross Paris and pick up the same train again to Bayeux.
Eurostar under the tunnel is so much faster and easier and often cheaper if you get tickets well ahead. It is reliable too and not affected by weather until real big storms stop everything.
price Eurostar, comfort Eurostar by a lot, convenience Eurostar by a mile, travel time Eurostar by a country mile, experience value don't know what that is. Seeing the White Cliffs through a rainstorm or fog?.