Stonehenge

You will find numerous past threads on the subject if you use the search feature.

Trained from London ( Waterloo station ) to Salisbury then took a bus to Stonehenge.

You can read about my trip to Stonehenge in my trip report:

Today was our big Stonehenge day. This is something my sister and
brother-in-law really wanted to see. I had read about the Stonehenge
Tour on the forum and it really worked well for us.

https://www.thestonehengetour.info/

It involved walking to the Waterloo train station to take a 1.5-hour
train to Salisbury. I had booked us open return train tickets in
advance. There are no seat reservations on this particular train. Then
right outside the Salisbury train station, the bus picked us up and
took us to Stonehenge. The bus ride is not time specific, so you can
take any bus you like. We took the first one at 10 am. The check-in
process was a little slow as it seemed most people had not purchased
tickets ahead of time. (I bought our bus and Stonehenge tickets ahead
of time from the tour website linked above.) The bus dropped us off
near the visitor center where we had to show our tour tickets to get a
Stonehenge ticket. We then took the free shuttle out to the site. We
had a fantastic day!! Blue skies!!!! It was not crowded at all. And
much to my delight, there were sheep all around. I had originally
planned for us to see the Salisbury Cathedral afterwards, but it was a
long day and so we decided to just head back to London.

https://community.ricksteves.com/travel-forum/trip-reports/10-days-in-london-our-first-time-a-trip-report

I went with London Walks but I'm not sure if they're still doing that day tour. It was great! But I think it's pretty easy to do on your own.

Stonehenge seems to be much more popular with North Americans than British people. I have never had a particular desire to go there - I have seen other smaller sites across the country. Others often say that nearby Avebury is more atmospheric as you can walk amongst the stones, which you can’t do at Stonehenge. All this is to get you to think whether you really want to go.

If you do have an interest in pre-history and want to go then it’s easy to do yourself as a day trip. Take the train to Salisbury and there is a bus from the station which will take you to the site. Make sure that you leave some time to see Salisbury and it’s cathedral.

“Stonehenge seems to be much more popular with North Americans than British people. I have never had a particular desire to go there - I have seen other smaller sites across the country.”

There just aren’t the Neolithic sites in the US as in UK, France, etc. so they are a novelty to most Americans. The closest one to me is a rock shelter with ceiling soot dated to 9,000 years ago that’s been backfilled to protect it. The next nearest one I know about is fairly remote and up at about 9,600’ so more difficult to get to than the sites I’ve visited in UK and France where you can often just park nearby.

I agree with taking the train then getting the local shuttle bus. To me the day trips that combine Stonehenge and Bath try to cram too much in. Conversely, doing a half day from London seems like too little time as there is a significant drive.

How many full days do you have in London?