I see 3-4 New Bharat coaches daily at UCGS, the only other one I see is Commonwealth.
From what I recall, the coaches are very expensive, something like £1500-£1600pa?

If it's Upton you're looking into, when you receive your welcome pack it contains a form to sign to give permission to share your details with other parents in your area, with regards to car share etc.

Car share can difficult due to all the possible pitfalls detailed by mad, not something to get into without very careful consideration.

If train is an option, UCGS and St B is walkable from the station but with heavy backpacks, Games Kit etc, it won't be fun!
IMHO, Langley is a bit too far to walk from the station, but I think there is a bus? but this will of course add more expense.

If your DC is going to use public transport then I would suggest getting a child's photo pass (it is like just like an oyster card).
As long as the child has credit on it then they can use the number 81 bus (travels between Hounslow and Slough) for free.

Both of my kids have a photo card and find it very useful. You do have to pay for the card (approx £15) as well as put some initial credit onto it, but as the card lasts 5 years it is effectively a free pass. The bus goes along the A4 so goes past Upton, St Bernard's and stops very near to LGS.

There are plans for oyster card readers at Slough train station which means a photo pas would give your DC cheap travel - just make sure it is topped up for the train part.