THINKING of Christmas gifts is hard. One tends to improve, revise, embellish and add to it all the time. But here's a unique gift idea for those with a budget: The seven-day (April 4-10, '99) English Country House program in Oxford, England.

April is just right. England is always beautiful but somehow better in the early spring.

The University of Virginia sponsors the program. It checks your soul into Trinity College (founded in 1555) in Oxford on Easter Sunday. Your body will be assigned to private quarters with an attached sitting room for a stay ''up at Oxford'' where all participants will live ''in college,'' relax in its gardens and courtyards as a prelude to meals served ''in hall'' at Trinity College.

If you are one of those special people interested in the English country house, then listen to this: ''The event presents informative and descriptive lectures by faculty from Oxford and the University of Virginia who have expert knowledge of the houses and gardens to be visited. It gives an opportunity to tour Oxford, showing the growth of the city and university through the ages by tracing the development of the medieval courtyard house.''

The highlight is getting a glimpse of the British aristocracy, ''seeing its evolution from the Middle Ages to the late 19th century, and observing the political, intellectual, economic and social changes that has shaped its development.'' Also, take a look at how the aristocracy lived and how it continues to live today. Walk through their country ''houses and gardens.'' Here are some samples:

Medieval castle

Broughton Castle, a moated medieval castle, is on an island at the center of a 3-acre moat. A bridge crosses the moat leading to the castle behind an embattled gatehouse. It is the ''house'' of the Lord and Lady Saye and Sele. Their house has not changed much since it was enlarged in the 16th century. It played an important part in the English Civil War when the 8th Lord Saye and Sele clandestinely met with oppositionists at Broughton to turn the tables on Charles I's efforts to rule without Parliament.

Canons Ashby House is an unusual Elizabethan house built in the 1550s by the Dryden family. It remains as it has always been since 1710. Leaded windows accentuate the rough and irregular walls of the house that surround a cobbled internal courtyard.

This large country ''house'' has one of the few 18th-century formal gardens in England set amid 70 acres of parkland. One of the few privately owned churches in England is in the property that once was an Augustinian priory.

Honington Hall, ''the perfect English 'country house': dignified in conception, comfortable in scale, and secluded in a park, with the village church almost at the door and the River Stour flowing just below the garden,'' wrote one author. The manor was built in the 1680s for Sir Henry Parker, a wealthy London merchant.

When modified in the first half of the 18th century, its interiors were fitted with exceptional Georgian plasterwork. It has the only domed octagonal saloon in England. Honington Hall remains in private hands, closed to the public, but special arrangements to visit it have been made.

After describing ''a drive down a pretty country road at the edge of the Cotswolds''--a bit of understatement in the announcement reads, ''After lunch at a country pub the afternoon belongs to Blenheim Palace, standing proud and imperious.''

Blenheim is indeed an international landmark. Now the home of the 11th Duke of Marlborough, Blenheim is one of the greatest baroque palaces in Europe. It was built for John Churchill, the 1st Duke of Marlborough. Overjoyed at his 1704 victory over the French at the Battle of Blenheim, the nation granted its vast acreage as a token of its appreciation.

Designed by John Vanbrugh, the top architect of the day, the immensity of the palace rivals Versailles. The leading specialists who were involved in its embellishment were Thornhill and Laguerre for decorative painting and Grinling Gibbons for stone-carving.

The gardens are magnificent. Henry Wise, Queen Anne's gardener, designed the original walled garden. Achille Duchêne subsequently added the Italian Garden and the Water Terraces. Landscaping to unify more than 2,100 acres of parkland was done by ''Capability'' Brown.

Waddesdon Manor is a jewel reminiscent of the French Renaissance-style chƒteau in the Loire. Built for Baron Ferdinand de Rothschild in 1874, the house contains his superb collection of 18th-century French decorative arts: French Royal furniture, Savonnerie carpets, porcelain from SŠvres, portraits by Gainsborough and Reynolds.

A collection of paneling originally carved for the great 18th-century houses in Paris is reassembled at Waddesdon, where rooms are reproduced as they had been during the reigns of the various Louis of France. The formal gardens around the house date from 1874-1881. Beyond that, the informal gardens ramble over the rest of the property.