You're tracing a route that doesn't really lend itself to a central base. On your first day, if you're leaving from Windsor and stopping to see Stonehenge, you will not have time to do justice to St. Michael's Mount. Note that Stonehenge is not just a matter of parking your car and having a look at the henge -- the car park is next to the (excellent & large) visitor centre, with indoor and outdoor exhibits, and you can either walk or take a shuttle bus about 1/4 mile to the henge and back. No matter what route you take from Stonehenge to Cornwall, you'll be mostly on secondary roads so your average speed will not be much more than 40 mph. Taking that into consideration, your first night might be in Truro, the only city in Cornwall.
Next morning, head for St. Michael's Mount, then on to Penzance, Mousehole, St. Ives. Well, I guess you could return to Truro for the night, but if I were you I'd stay in Tintagel or someplace near it for the second night. Then in the morning you could walk around and enjoy the charms of Tintagel before you get on the road toward Bath.
In mid-October the days are relatively short, so you'll want to make the most of the daylight while it lasts. Also be aware that some seasonal places will have closed. St. Michael's Mount castle is open (except Saturdays), but the gardens appear to be closed for the season.
