Public television this evening is affording us a pleasurable foretaste of what should prove to be a major high-light of the Washington dance scene this summer. "An Evening With the Royal Ballet," which airs on Channel 26 tonight at 8 o'clock, gives us an hour's sampling of England's balletic pride and joy, the same troupe that will hold forth at the Kennedy Center Opera House for two weeks beginning July 14.

The sample is not precisely representative. Tonight's program is an abbreviated version of a longer one originally taped several seasons back as a co-production of the BBC and Metromedia, in fact during live performance of the company at Covent Garden. What's been retained for the PBS telecast is perfectly respectable repertoire, but decidedly on the fluffy side -- sort of a "pops" ballet assortment. All the same, we do get some revealing glimpses of the elegance, poise and stage presence that have long been the Royal's trademarks.

The program's one complete item is Kenneth MacMillan's "Elite Syncopations," a self-consciously giddy ragtime potpourri that afirms the choreographer's genuine wit and craft but wears out its welcome with several too many "zany" numbers. Still, it's performed to sly perfection, and rambunctious Wayne Sleep, drily saucy Monica Mason and svelte Jennifer Penney are shown off to fine advantage.

The rest of the hour is given over to a pair of pas de deux: the maypole duet from Frederick Ashton's version of "La Fille Mal Gardee," danced by Wendy Ellis and Michael Coleman, and the "Bluebird" sequence from "Sleeping Beauty," with Jennifer Penney and Wayne Eagling. Ellis and Coleman have just the right sort of crisp ebullience for the Ashton, and the duet itself is a little masterpiece of buttery charm. As the "Bluebird" pair, Penney and Eagling lack ultimate sparkle, but the line, phrasing and technical finish of their performances are exemplary