Eccentric splash and . . . . . . practical dash. London designers show flair for fantasy, but their clothes are wearable, too

ECCENTRICITY has never been missing from the British scene, in society or fashions. Indeed, there have been moments when we seem to have rather too much of it. The best moments come when the balance is right. And this fall's British fashions seem to have achieved it with the proper mix of fantasy and practicality.

John McIntyre's idea of building his collection around the personality and times of Vita Sackville-West could have tilted that balance. That it didn't says a lot for McIntyre's own attitude and the experience he must have gained from working with Luciano Soprani in Milan.

McIntyre took his inspiration from the William Strong portrait of Sackville-West painted in 1918 called ``Lady in a Red Hat.'' He asked Viv Knowland, one of the bright new milliners, to provide him with similarly broad-brimmed hats to wear with his collection of ankle-length coats, jodhpurs, and pantsuits in wonderfully rich jacquard weaves, tapestry, leather, and fur. The overall effect was show-stopping, yet take the items one by one and you have effective, wearable clothes. Genuine eccentrics, howeve r, can wear the McIntyres as he showed them.

Sara Sturgeon is another pleasantly zany designer at her best this season. While Sturgeon was studying at the Royal College of Art, she worked with Daniel Hechter in Paris and won a design competition working for Harvey Nichols. Since she left in June 1984, she has been working on her own collection called Clothes.

Sturgeon likes working in unexpected fabrics, using cotton blended with wool and rayon, or rayon with linen. Her clothes are fluid, cut into asymmetric points and hemlines that reach the ankles. Yet, like McIntyre, she designs clothes that are essentially easy to wear.

All the designers, Jasper Conran and young John Rocha among them, have enjoyed playing with the combination of jodhpurs and riding jackets. Sometimes the jacket becomes a highwayman's coat, to wear good and long over a swirling skirt or trousers.

Looking back is still very much in style this season in London. But if you don't like the dandy look in brocade waistcoat and rich velvet jacket, then you can wear Emanuel, whose designers favor the Jean Brodie look -- long, narrow, low-waisted styles in silk crepes or lightweight wools.

No doubt about it, the British secretly do enjoy dressing up.