Day trip from London to Stonehenge

It sounds like you've already secured a tour (cost) for Stonehenge, but one effortless way to get there and Windsor Castle is via Grey Line. They have a full-day itinerary which first goes to Windsor, then to Stonehenge, and finally into Bath. And the best part of all is you don't have to concern yourself with train schedules/stops or an expensive taxi to see the different venues. Windsor's stop is about 1.5 hours (includes tour); at Stonehenge you get to spend about an hour; and Bath allows for a 2-2.5 hour stay, which is perfect for seeing the Abbey & Roman Baths and wandering into the shops/pubs.

Thank you very much for respond. We don't want to do group tours. I was able to schedule early access to inner circle through English Herritage. And we not planing to visit Bath as well. Besides Long Barrow not even included in any of the trips. That why we prefer to do everything on our own schedule. But like I mentioned earlier, train is not an option because we need to be there ( Stonehenge) 7:30. And we weren't planing to rent a car till few days later ( we'll be staying in London for 5 days and then travel though England and Scotland by car). But unfortunately that particular day was only an option to get inside the circle and it will be in the middle. So I was thinking that private driver could be cheaper then taxi.

We did the stone circle access with our own rental car. We drove there the day before so as to minimize the risk of taking a wrong turn on the all-important morning.

We stayed overnight in Newbury, which is not all that close to Stonehenge (variety of reasons involved in that decision). Nearest attractive town to stay in, as you noted, is Salisbury. There is a motel right along the A303 at Solstice Park but it is a modern business-type place, not quaint or anything. Our stone circle access was quite a bit earlier in the morning, which allowed us time to have breakfast at the Park House Motel in Cholderton/Amesbury. The motel is a bit worn, but great service and close to Stonehenge.
https://www.tripadvisor.com/Hotel_Review-g1088512-d632296-Reviews-Parkhouse_Motel-Cholderton_Wiltshire_England.html

I suggest after your 8 AM visit, go over to Woodhenge and Durrington Walls, just about 1/2 mile to the east.

Like I sad before this day is in the middle of our stay in London. If we spend night in Salisbury/ Stonehenge area we ether have to pay twice for hotel, or check out with luggage in London and then go back and check in again. So it's a huge headache. Besides, 5 consecutive nights in hotel much cheaper then 2+2. We thought about renting car night before, but there is absolutely no parking in our hotel vicinity. So we'll still need to take taxi to get to the car @ 5:30 am. Considering all costs (rental+parkings+taxi) it probably will be same amount as private driver for one day.

Is this the sort of service you are seeking? --- http://davidsdaytours.com/

I have no idea how it compares to a taxi cost wise

Have you checked with your hotel? They often will have either a company to recommend, or people they with work with to drive you.

Perhaps not useful for you, but for others reading this: I think you're trying to fit a square peg into a round hole. Stone circle access is very, very special. You went through the trouble to book it. It will be a once in a lifetime experience. Yet you're not willing to plan your trip around it -- instead you are trying to fit it into a predetermined stay in London. Your choice.

We spent a couple nights at the Solstice Park Holiday Inn earlier this year (hurrah for free nights from award points). It' a perfectly nice modern 3 star hotel that's easy (save for an awkward traffic circle) to get to from the main road.

My trip, I'd see Stonehenge, and then make a day of the area, stop at Old Sarum after that, and then use the park and ride lot pretty much next to Old Sarum as a spot to catch a bus ride into Salisbury. See the cathedral because I'm a cathedral junkie, check out a better copy of the Magna Carta than they've got at the British Library, and wander around a bit in a lovely little place.

To continue what epltd said, you have kind of created a dilemma for yourself.

There are many, many car services around London that could take you to Stonehenge, but this is not a short drive. Are you aware of the distance involved?
You would have to leave out of London at God-knows-what-hour. 5 AM possibly? Meaning, you'll have to awake, dress, eat beginning maybe 3:30 AM?
I would start with the car service "Just Airports" and see what price they would quote you for the trip, and how early they could pick you up that morning.
https://www.justairports.com/tour.php
You could arrange for them to take you (after Stonehenge) to Windsor Castle and drop you on your way back; then when you are finished there train back to London.

The other option is to check on times for a train to Salisbury leaving from London at some very early time that morning, then take a taxi (pre-booked) from Salisbury to Stonehenge.

Another way to do this is to bite the bullet (I know you don't want to) and book a stay at a hotel near Stonehenge, and stay there the night before. I know, I know, you're already paying for a hotel in London for that night. But this may be the price you have to pay for being able to get to Stonehenge on time without making yourself miserable getting there.

By the way, the car service/taxi ride from London to Stonehenge may be as expensive as paying for an "extra" hotel room for one night.

Another option is to rent a car yourself the day before, drive to Salisbury, spend the night, drive to Stonehenge in the morning. Then you will have your car to drive to West Kennet Long Barrow and/or Windsor Castle, as you mentioned.

Good luck to you. I realize you are trying very hard to work all this out, and I hope it does work out for you. Have a great trip.

Thank you very much to all for responds. I got a lot of useful information. I got quotes from few car services companies and will make reservation with one of those. We still don't want to ether rent our own car just for that day or stay in Salisbury vicinity. Buy the way waking up @ 4-4:30 in the morning wont be any problem because we'll still have jet lag.
Thank you again.