Was considering because of the short time we have and the afternoon 2 hrs out, 2 hrs there and 2 hrs back seemed like a good deal for an afternoon/evening tour.
With so little time in London, I would stick to spending time there, rather than wasting hours in transit to and from Stonehenge, which is a complete anticlimax. I would be surprised if your door to door travel times aren’t closer to 3 hours each way, allowing time for security checks.
Since she's a "local" I'm sure Jennifer is accurate about the approximate drive times - 2 hours sounds very short unless it's in the middle of the night. I was on a Road Scholar tour a few years ago which had us picked up by bus from our London hotel to head to the SW of England and it literally took an hour just to get to the outskirts of London.
I also agree with just 2.5 days in London, I'd save this for your next trip.
It's hard not to have "travel creep" which is when everything looks wonderful and you want to do it all. There is SO much to see in London that 2.5 days (and one is an arrival day from the US or Paris?) is not really enough just to see the main sights there.
You'd spend most of your day on a bus on a motorway and in London traffic.
Stonehenge is worth a stop if you are in the area for a few days or driving through.
Stonehenge was very anti-climatic for my husband and I. VERY touristy and extremely crowded with buses constantly coming and going; you can not get close to the structure and have to maneuver around hundreds of people taking selfies. It was interesting, but we found many other similar structures that were much more accessible, absolutely beautiful
and way less crowded...if you get a chance to go to Kirkwall, in Scotland, you will be blown away!!
Stonehenge is one of the very few things Jennifer and I disagree upon. She doesn't find it interesting whereas I find it fascinating. The reason I find it fascinating is because I'm very interested in anthropology and the Neolithic era. I don't go to Stonehenge because it's on seemingly everyone's checklist, I go because it fascinates me.
From a previous post it would appear that you have 2 1/2 days in London so unless you have a real interest and burning desire to visit Stonehenge I would use what precious time you have exploring London.