Your schedule falls apart on Day 3. There's enough listed there, on Day 3, to fill 4 days of a normal vacation.
It is simply impossible to do all of what you have planned for your entire England portion.
You say "London and Port Isaac is (are) on our bucket list."
"I am going with some people who want to see Stonehenge"
I suggest that you put your "bucket list" items first.
Day 3: Early train from London to Port Issac (train station Bodmin Parkway). Stay that night at a B&B in Port Issac.
Day 4: Early train to Salisbury. Take the Stonehenge Bus out to Stonehenge from the train station. Then bus back to Salisbury. Then train back to London. Sleep in London that night.
Day 5: Train to Paris.
I am repeating Lola's first paragraph here:
"London Paddington to Bodmin Parkway is 4 hours on the direct train. We did this several years ago and lived our time ( 2nights) in Port Isaac. We took a taxi ( pre-booked) from the train station to our B and B in the village for £30."
Lola is giving you great advice when she tells you that you can take the train out, taxi to your B&B, and not have to use a car.
Just so you know...Port Issac is a really, really long way from London. I don't know that the pay off will be great enough for you to warrant the amount of trouble it will be.
Skip Oxford and the Cotswolds. You do not have any time at all for it.
Do NOT pick up a car. A car slows you down. The train is fast. And you don't get lost.
You say this is your first time to London or England. Even more reason not to drive while you're there.
It's okay to leave some of your bucket list stuff for the next time you visit England. Since it's your first time over there, you may want to spend all your time in London. There's a lot to do in London! And you really don't have a lot of time.
As an alternative plan, if you spent all your time in London, you could do one day trip out to Salisbury/Stonehenge by train.
It would be much less stressful to do this and forget all the other stuff and leave it for another longer trip to England some day.