The birthplace of Winston Churchill (whose mother was American, btw!), Blenheim Palace is a 20 minute trip
from Oxford, and about an hour and half from London or Birmingham.
Built as a gift to the first Duke of Marlborough, John Churchill, the palace celebrated the British victory over the French in the War of the Spanish Succession at the Battle of Blenheim in 1704. It’s a rare example of English Baroque, and like many grand houses, fell into disrepair until the 9th Duke of Marlborough married the American heiress Consuelo Vanderbilt.
The area was once the manor of Woodstock, formerly a royal deer park. Between 1554-55, at the age of 21, Elizabeth I was imprisoned in the hunting lodge by Mary I.
The palace history is a favorite among architects as an example of what happens with difficult clients, and they note that the first architect, Sir John Vanbrugh, was foiled at every turn by the Duchess of Marlborough, who wanted grand + cosy + low-cost, and they eventually fell out. The palace was finished by Nicholas Hawksmoor.
The current duke and his family still use it as a home.
It does not feature in Downton Abbey, though tours now hail it as “the real Downton Abbey” and include it on itineraries.
Travel info is here: http://www.blenheimpalace.com/planyourvisit/getting-here.html