We made the trip there this last Monday for the Spring season.
A few points;
We did it DIY Vs using Viator from the Vitoria coach station in central London to provide more flexibility and an alternative to $250/person pricing. (An individual entry is about $35)
We made our reservations about 2 months before our visit.
We booked the 1230-230 entry slot to allow us time to get there, time for the gardens, and time to get back to London.
We booked train tickets from London Paddington to Newbury for $33 each round trip on Trainline App.
So that brings us to getting the 8 miles from the Newbury station to Highclere. Uber is problematical. It got us to Highclere (for only 20 pounds) but not back from Highclere. The Highclere ticket office can call you a cab back to Newbury station at castle closing time. Count on a charge of 35-50 pounds but the common cab is an eight passenger van ……..so try to share costs. We found folks willing to split the charge.
The castle was awesome as was the grounds and the Egyptian exhibition. I highly recommend the trip.