All pre-booked Highclere Castle (fictional Downton Abbey) tickets for this summer and most scheduled tours from London sold out some time ago, but good news there is an easy way to get there from London and get tickets on the day
Getting to Highclere and Daily Tickets
You take a train to Newbury and from there a taxi for the 7 miles to the Castle (taxis will be waiting when you arrive at Newbury station and you can pre-book the same cab to pick you back up at Highclere later)
If you turn up at Highclere in good time (about 9.15/9.30am) before the start of the morning opening you will be able to buy tickets from the generous daily allocation
Arriving at that time you will likely be first in the queue when the ticket office opens
Overall this is a very straightforward way to reach Highclere and a lot cheaper than taking a fixed tour
Alternative out of London Coach Tours
Downton Abbey Tour with Highclere Castle & Filming Locations
Downton Abbey Filming Location, Blenheim Palace & the Cotswolds Tour with Lunch