Driving Bath to Highclere Castle and on toward London - am I crazy?

Ever driven on the opposite of the road? And if you can get tickets is an understatement. IF you did get the tickets and did drive I'd drop the car in Salisbury ant train to Waterloo station in London from there.

Just rent a car and do it! Once you pick up the car drive around a quiet neighborhood until you get the hang of it. It doesn't take very long even if you end up with a manual transmission. As long as your daughter can navigate you'll be fine!
Highclere is far less crowded than it was at the height of the Downton Abbey craze...although they are only open for 7 days in May. I just popped over to their website to check. They're open on May 1, 2, 4, 18, 28, 29, and 30 and it appears they already sold out except for the 4th and the 18th. Not cheap though...since these are special home/garden tours outside of the regular tour season they are 90 British pounds per person. Yikes!
The previous poster's idea of dropping the car in Salisbury is a good idea.
Hope it works out for you to visit...it's such a lovely place!!

In answer to Claudia's question - no, I have never driven on the left. But at least we will have been in the U.K. for almost 2 weeks by then and should be somewhat more used to traffic patterns. Salisbury does seem to be a bit far and in the wrong direction - I was thinking perhaps Reading or around Windsor. Or maybe Heathrow?

Good to know the tickets can be so pricey! I plan to start checking in early February for our June dates and will keep my fingers crossed. On the other hand there's always plenty to see in London...

The shortest route between Bath & Highclere is to head E on the A4 towards Chippenham - which has MANY roundabouts! (You stay on the A4 all the way to Newbury and then loop onto the A34 heading towards Winchester. (You will soon see signs for Highclere).
However, mid route - not far E of Calne - is AVEBURY = well worth a look. Probably best to turn in the car in Newbury & then train to London.
https://www.nationaltrust.org.uk/avebury
http://www.english-heritage.org.uk/visit/places/avebury/
Also in this area:>http://www.english-heritage.org.uk/visit/places/west-kennet-long-barrow/
http://www.english-heritage.org.uk/visit/places/silbury-hill/

Try and follow the road on Google Earth so that you know what to expect. (You will pass through Marlborough). (An alternative longer route is to the north - using the M4).

Don’t forget, that if you wish to do day trips from Bath by train, then Salisbury can be reached in 1 hour and has a wonderful cathedral. (Shuttle buses also operate to Stonehenge).
http://www.visitwiltshire.co.uk/salisbury
http://www.english-heritage.org.uk/visit/places/stonehenge/

You can also reach in 1 hour by train Cardiff, the Capital City of Wales. www.visitcardiff.com
www.cardiffcastle.com

The regaular tickets to see the castle and gardens are £15.00. They haven't posted the dates for June yet.

There are numerous coach tours leaving from London and going to Highclere Castle. Some of the tour bus companies buy up large blocks of tickets to Highclere. This is one reason tickets can be hard to get during prime tourist season. May be easier to take one of these tours, which includes the transportation and all entry fees.
https://www.premiumtours.co.uk/category/highclere-castle-downton-abbey-tours
https://www.internationalfriends.co.uk/downton-abbey-filming-locations-and-highclere-castle.html
https://www.londontoolkit.com/tours/premium_downton_abbey.htm

I would take the train from Bath to London, then take one of these tours.
You're going back to London anyway, from your original post.
Better to do it the easy way rather than the hard way, IMHO.

Reading or Newbury will be the most sensible place to return the hire car. Don't attempt to drive in Windsor - the traffic is hideous! Congestion is bad around Heathrow and then you will get stung for high train travel into London. The one way drop fee will be high whatever you do. Buy your rail tickets direct from the operator gwr.com.

Only attempt this is you can get tickets for Highclere. I assume you will also want to go to the exhibition, which means that tickets are a whopping £22 per person and from what I can see, there are no tickets left for May, so look out for June tickets. You only get access to the castle for either the morning or the afternoon - you would need afternoon tickets of coming from Bath. There is still high demand for tickets here, which is why they can get away with charging about double what the National Trust would do for something similar - it's all about supply and demand!

" I have never driven in the UK, but the drive looked a lot easier than train or bus, especially with our bags. "

It isn't. For just one day, I wouldn't try it.

Thanks to all for comments and suggestions! We have been to Avebury and Salisbury and I think will find our three days in Bath are full, so we're not planning any day trips. The purpose of this HIghclere trip is partly to see the location [including Capability Brown's landscape] and party to get us from Bath to London efficiently.

However, having looked at the price for a one-way car rental (at least $150), I am leaning towards saving Highclere for some future trip. Maybe it won't be as popular in a few years? I will check in early Feb for possible June dates but will probably end up crossing this off our list for this trip.

I realize that Highclere has the double attraction of Downton Abbey and Capability Brown but, on your way from Dorset to Bath you will be passing Stourhead and Longleat House. Longleat is another Capability Brown landscape. Stourhead is not attributed to Capability but it has one of the most beautiful landscape gardens that I have ever seen. You did not mention how you were getting from Dorset to Bath but L and S are both within easy reach of Bath if you should decide to rent a car for a day for some landscape immersion.

"The purpose of this HIghclere trip is partly to see the location and partly to get us from Bath to London efficiently."

Getting from Bath to London efficiently (since you used this word) is best done by train. Quick and easy.

Getting from Bath to London by car is not doing it the fastest nor the most efficient way.

Adding Highclere to the journey does not make it any more efficient to take the car from Bath to London.

I agree that you may want to skip Highclere for this trip. It complicates your trip, rather than making it easier.

Thanks, all. We have decided to skip Highclere Castle this visit.

Trinitony, your suggestions about Longleat and Stourhead are very helpful! I will look at them and also check other National Trust sites closer to London that might be interesting to visit. So much to see and do...

You probably are aware of this map showing the location of landscapes designed by Capability Brown - but just in case you are not, here it is:

http://www.capabilitybrown.org/map

You will see there are many properties close to London where you will be ending your trip.