First of all, I want to thanks everyone who asked and answered so many questions previously. They've been very helpful.
Next, a little background information to help give context. Next March we'll be taking a family trip, flying into London. We'll have 8.5 days on the ground, not including our departure day. There will be 9 in our group (3 grandparents 65-70 yrs, 4 kids 5-13, and my spouse and me). This trip grew from my family just in England to include the grandparents and Paris. 3 years ago, my wife and I did 3.5 days in London and a week in Ireland/Northern Ireland, so we have some lay of the land.
So, on to the questions.
My wife insists on seeing Cornwall and I've been tasked with making it happen. She and the grandmothers all love Poldark, so she wants to be able to have them all see the area. I know it's a long drive and going to see it without spending extended time there means you drive for the majority of two days and have very little time to see things. We live in the Western United States and multiple times a year drive extended distances for vacations/visiting family and so my wife insists we can handle it.
So my questions are first, how to execute this plan and second, what would be an alternative location that would be more reasonable to visit that I could convince my wife of (she wants at least a day or two that is her choice and not just seeing the standard things in London and Paris).
Current plan is that our flight (already purchased) lands at 3:20 on a Sunday. We intend to take the HEX to Paddington and grab a train to Bristol and spend the night there. The next morning we'll rent a 9 passenger van from Sixt and drive to Bath, spending the duration of the morning there. From there, we'll drive down to Stonehenge and spend an hour before heading to the Newquay area, arriving in the evening.
The next morning (Tuesday) we'll get an early start and go over to St Agnes head and walk along the shore. There's a chance of doing something else as well depending on what my wife finds in her research. We'll leave the area by 11 to get back to Heathrow by 4, catching the 6:01 Eurostar for Paris.
So, my biggest concerns about this (other than acknowledging that it doesn't have sufficient time for food and breaks and gives us almost no time in Cornwall) are:
Will we be able to reliably rent a 9 passenger automatic van in Bristol? Sixt says I can rent one there (part of the reason for Bristol instead of going to Bath directly), but I'm worried that they won't actually have one available and then what? No one else in the group should/wants to drive.
What else am I not accounting for? Normally I expect that I can look at Google maps, add a half hour for a half day drive or an hour for a full day drive and be good. Google calls the drive from Newquay to Heathrow 4 hours and 10 minutes. I figure budgeting 5 hours gives me time to drop the rental car off after filling it up with gas and make the hour tube ride to St Pancras with an hour to get onto the Eurostar for Paris.
What sort of alternatives can I offer my wife that don't require a full day's worth of driving on an 8 day trip? Something outside of London, preferably nature based. Something that would be a good alternative to seeing the Poldark area, but maybe similar type of area.
Right now I'm trying to get tickets purchased, and rooms reserved. Planning for a group of this size has me worried about finding good places to stay and buying advanced tickets to keep things as affordable as possible. However, we haven't purchased/reserved anything other than plane tickets yet and so we can still shuffle things around.
After the Cornwall excursion, it's 3 days in Paris and 3 in London which I think I have a handle on.
Thanks for reading the rambling. I've been impressed with the quality of those who help to answer questions on this forum, and am hoping to gain some insights to have a great trip.