Is it possible to see both Winchester Castle and Stonehedge as a day trip from London on your own?

Oops! I meant Windsor Castle and Stonehedge.

Golly, I spent from 5:15 to 6:45 on Google trying to find a way to do it. Only the clink of ice cubes interrupted my quest.

Depends if you have a car. Stonehenge (not hedge) is quite remote, being in the middle of Salisbury Plain. Windsor Castle is easily reached by rail from London. The station is right opposite the entrance to the castle. Tickets can be booked on line in advance and so saves the queues (www.royal.gov.uk).
I am not aware of any tour that would take in Windsor and Stonehenge in the same day. I would suggest hiring a car to see Stonehenge.

omas, there is a bus tour which combines Windsor, Stonehenge and Bath for 59 pounds. The website is:

http://www.premiumtours.co.uk/ourtours.html

The only other direct way to Stonehenge I could find was from Salisbury by bus.