I grew up with a father who LOVED watching Stonehenge documentaries, so when I went to England the first time, you better bet I drove the 3 hours from where we were staying (Dover) to see the mythical and mysterious site. While it didn’t quite reach the exuberantly high expectations set out by hours of documentary watching, it was worth doing (if you have a free day in England). Just be warned you can’t get close to the monument and it will be a crazy tourist attraction busy.
As for the practical accessible aspects, Stonehenge is pretty good. I’d give it a 7/10 as there was an accessible bathroom, ramp access, and a paved path for half of the site but grass for the other optional half. Their website has a great description of access for patrons of various abilities that you should check out here. Note: since I went to Stonehenge there have been some renovations and you now take a shuttle from the parking lot to the site. According to the Stonehenge website, the shuttle is wheelchair accessible.
Halfway around is a nice paved trail. The other half (if you choose to do it) is mowed grass. It’s mostly flat (some ditches but nothing unmanageable). However, it is a fairly long way around (I was really slow cause I couldn’t stop taking pictures!) and my arms did get somewhat tired by the end. You might want to consider putting push handles on if you have removable ones. There is one ramp you use to go through the tunnel to get to the site, which is somewhat long (15 meters) but a manageable grad. There is an accessible bathroom in the tunnel near the souvenir store. Helpful hint: We went quite early in the morning (about 9:30 am or 10 am when it opens), which was the way to do it since even then it was busy and as we left it was an absolute zoo.
Accessible Bathroom = Yes
Smooth Paved Paths = Flat, 1/2 excellent and 1/2 grass
Ramps = Yes
Accessible Public Transportation = Yes
Getting Here: Annoyingly, Stonehenge is slightly difficult to get to. It is a 2-hour car ride from London (I recommend driving). There can be some traffic, as you get closer to the site so give yourself a little extra travel time. I highly recommend bringing your disabled car tag (my Canadian one is recognized in England) as there is lots of disabled parking and otherwise, the busy parking lot can mean you have to walk/ wheel quite a ways.
Prices: See prices here for student, senior, and family rates. Wheelchair users can bring a companion in for free. This is fairly common in England and worth asking at any tourist attractions. You usually don’t require any sort of proof, just tell them so and so is coming in as your companion.
You can also train to Salisbury and bus from Salisbury to Stonehenge. From what I can gather online it sounds like both trains and bus are wheelchair accessible. The trains are accessible; I’ve taken them many times, except you need to book ahead of time if leaving from a smaller station. Likely, if coming from London, you will leave from Waterloo station, which is a major station. You can still book ahead if you prefer or just go to the information desk and they will arrange for someone to put the ramp out for you. Try and get there early, as they can be a bit funny about making you wait for a later train if you cut it close. If you are phoning ahead I highly recommend putting some money on a Skype account. It is usually cheaper, you don’t have to worry about roaming charges or running into an area without service as long as you have an internet connection. When buying train tickets in England make sure you go to the desk (instead of using the machine) and ask for the price for people that stay in their wheelchairs. It is a 34% discount for you and your companion but usually, they won’t give it to you unless you specifically ask for it.
From what I can find online the bus from Salisbury to Stonehenge (Salisbury Reds bus service) is accessible. Unfortunately, I haven’t taken it myself so can’t talk from personal experience. If someone has used the public transport route to Stonehenge please tell me about your experience in the comments! There are also options of taking coaches from London to Stonehenge but I have yet to find one that is accessible. Again, if someone knows of one let me know!