Finally! An itinerary!

I won’t be the first to ask but why Dover? White Cliffs aren’t what they seem to be and the town isn’t interesting at all.

I would drop the car off in Salisbury after seeing Stonehenge. Visit the Salisbury cathedral, have a late lunch then train to London.

In London don’t miss the Treasure Room at the British Library then walk towards St Pancras train station and explore the adjacent Granary Square and Coal Drops Yards which are behind the Station. This whole area has gone through a massive revitalization in the last 7-8 years and it’s impressive. Then treat yourself to tea or cocktail or lunch at the gorgeous

( albeit pricey) Booking Office Restaurant In St Pancras. It gorgeous. Walk into the lobby of the hotel and before the check in desk veer right. That will take you into the Booking Office.

Or if not your style their is a Dishoom at Granary Square. Fabulous food.

I’d also make certain you see at least one theatre performance while in London.

Lastly, maybe spend Friday at Portobello market or Borough Market or enjoy a wander around Brick Lane. Lots of eye candy.

I would stay in the charming Cotswolds; after, you can drop off your car at LHR and take a train into London.
As first time visitors to London, yes to Tower of London. I would add the inside tour of Buckingham Palace as the number one place to visit, timed entry tickets; Churchill's War Rooms; Westminster Abbey, many excellent museums( free) to choose from too.
Get Buckingham Palace timed tickets in advance, online from the Royal Collection Trust.
Day trip: Windsor Castle, by train, one change. Tickets in advance.
I do not understand Dover, White Cliffs at all either. I have seen them from above flying from France to London.
Last time we were in London and at Windsor, the lines to enter for all the above sites were very long. We had bought tickets in advance online and went to the front of the line.

Looks a bit hurried - I'm sure many here will say slow down - but I usually hurry too.

Just one suggestion: on your way from the Cotswolds to Stonehenge, be sure to stop at Avebury. I enjoyed Avebury 100X more than Stonehenge (Stonehenge actually left me feeling depressed over the scale of modern mass tourism and what it has done to the experience of visiting special but all-too-famous places). At Avebury, there were no crowds, no fences, no tour buses, no restricted entry times, no...well, you'll see when you get to the (admittedly necessary) Stonehenge visitor center. I'm sure others would disagree, but Avebury turned my Stonehenge frown into a smile.

Dover area because that is where my family is from, it's really Ramsgate. I'm toying with the idea of axing this also though...

If you decide you don't want to drive in the Cotswolds (though I don't know of any reason why you shouldn't), I enjoyed my one-day van tour run by GoCotswolds. It picks up at the train station in Moreton-in-Marsh. I think the Secret Cottage tour does, too. I stayed in Oxford for several nights, and it's easy to take the train from there to M-I-M for one of the Cotswold tours. More convenient for your current itinerary would be the MadMax Cotswold tours that depart from Bath.

You've mentioned a couple of pricey London sights that are on the current 2-for-1 list. The list of participating sights, and date exclusions, can change, but it would be worth checking your itinerary against the information on the website to which I've linked. Buy one/get one free is a very nice deal. I don't know how well this would work at the Tower of London, which I believe has very long lines. You might decide you'd rather just buy tickets in advance (assuming that you can: I haven't checked) and forego the potential savings to avoid time wasted in line. But I've never heard of a significant delay in the ticket line at Kew Gardens.

As I understand it, taking advantage of those 2-4-1 deals requires that each of you have either a train ticket into London for the day in question or an Underground travel card purchased at a regular rail station (not an Underground station). This subject has come up often on the forum; you can find more information by using the Search function.

It appears that you'll have several rail trips that can be much, much cheaper if you snag Advance tickets shortly after they go on sale; I understand that usually happens 11 to 12 weeks before the travel date. I recommend checking nationalrail.co.uk now to see which travel legs have prices that vary a lot, depending on how early you buy the tickets. (Compare prices for travel tomorrow to Advance ticket prices for May.). The savings can be huge--way over 50% for some routes. The Advance tickets generally tie you to a specific date and departure time. You may be able to make a change later, but there will be an administrative fee and you'll have to pay the difference between your original bargain fare and the current fare for the new ticket, so a last-minute change will probably be very costly. Best to be sure of your timing before buying any rail tickets.

Your plan makes sense to me. I would have been a Dover skeptic too but you have a good reason to go that way.

I haven't been to Avebury but others have shared David's observations. It would be relatively easy to work into your plan. And, if time allows, Salisbury's cathedral is well worthwhile. Good place to drop the car (Salisbury, not the cathedral).

Well, Ramsgate is completely different to Dover. Easy day trip by high speed train from London St Pancras International station.

The development that started a few weeks ago to try to support the non existent ferry company to take delayed trucks to France after Brexit has stopped again after the company going to run it was found to never have either owned or operated any ferries or ships, the money was grabbed back, and the plan halted.

Why I say all this is that the harbour area of Ramsgate will be quiet again for your visit. It has been derelict for some many years.

I'm glad that you got decent days and at a good time of the year, too!! Yay

If you feel you have enough time in London, I would keep the day trip to Dover (or Ramsgate) if you're interested in the family connection. You could combine it with a visit to Canterbury or Leeds castle - both worth seeing. If you do go to Dover, take a boat trip to see the white cliffs from the sea, the best vantage point.

Combine Ramsgate with historic Canterbury.https://www.canterbury.co.uk

I loved my day trip to Dover. The castle is quite interesting and the underground tunnels were also fascinating. Always interesting to see how different people react to different places.

We were thinking of doing Ramsgate and then Dover in one day, which is why it's a bit confusing, I didn't phrase it well. Thanks for the suggestions! Once I get the rough itinerary down, I'll start fleshing it out with sites to see.