Geoffrey Roberts' office is a short block from the Chelsea Embankment and the Royal Hospital, home of the annual Chelsea Flower Show. Not a part of London in which one expects to find a wine shipper. The office is in a townhouse, with a narrow garage occupying the entire ground floor. Lift the garage door and there, far from home, are familiar labels: Joseph Phelps, Dry Creek, Firestone, Robert Mondavi, Chateau Montelena.

Roberts is the leading shipper, or importer, of top quality California wines into Britain. Lest one get the impression that the size of the market could be judged by the storage capacity of the garage, he is quick to point out that these wines were for immediate delivery. There is a larger warehouse elsewhere. A slim, quiet-spoken young man, Geoffrey Roberts is synonymous with California in the London wine trade. Robert Mondavi has said, "We selected him because we needed a spark; someone who was not too traditional in approach." For Mondavi and his other principals, Roberts successfully combines his London trade background with an enthusiasm for the wines and the wine business of California.

Are the wines selling? "There is a great deal of interest from the wine trade itself, but the public is still getting used to the idea that America is a wine producing country," says Roberts. In the past 12 months, though, the competition has warmed up and, among others, the wines of Wente, Christian Brothers, Gallo and The Wine Spectrum (better known here as Taylor California Cellars) are now available in London. And the big companies are spending. Paul Masson has launched its carafe line with this slogan: "Ronald Reagan wasn't the only Californian to win the popular vote in 1980!"

By Washington standards, prices are high, thanks to duties and taxes. Duty on a bottle of table wine from a Common Market country is 70 pence a bottle ($1.40) and from a third country, including the U.S., is 80 pence ($1.60). Then there's a healthy 15 percent VAT (value added tax) slipped onto the total bill whenever the wine changes hands, from shipper to merchant to consumer. If the retailers and restaurants didn't work on smaller profit margins than their American counterparts, wine sales would have been even more affected than they have in this past winter of economic woes. There's no doubt the London trade is suffering, but those handling good quality wines, with long-established customers, are surviving better than those who rely on fast turnover and supermarket business.

For anyone visiting London who'd like to give English friends a California wine, vendors include: Stone's of Belgravia, Harrods, the Andre Simon shops and La Vigneronne. Owned by Liz Berry, one of six women masters of wine, La Vigneronne specializes in the wines of America, South Africa and Australia.

And, for any American in London who's just plain homesick, here are some of the wines available this summer. The '77 Firestone Merlot is about $9 plus VAT; '78 Trefethen Chardonnay is $13 plus VAT; '78 Ridge Paso Robles Zinfandel is $11.50 plus VAT; and Robert Mondavi's '79 Fume Blanc is $10 plus VAT. POST SIP

Bronx Cheer -- With a face as red as a . . . raspberry, I confess to getting a bit carried away in the July 5 column. Jean Danflou, the eaux-de-vie producer, uses abut 6 kilos of raspberries to make one bottle of framboise. That's about 14 pounds, not the overly fruity 66 pounds I quoted.