With the British pound at near-record lows last year, Harrods, London's largest department store, invited Americans to fly over to pick up bargains at its annual after-Christmas sale.

But this year, the pound is up, and transatlantic flights are not quite the bargain they once were. So, this week Harrods is using a toll-free, transatlantic "800" telephone number to try to lure American dollars to the store. Americans who use the number can buy cashmere sweaters and coats -- a Harrods specialty -- a week before the start of the store's annual sale.

American Telephone & Telegraph Co. said Harrods is the first store to use an international 800 number -- resulting in no charge to the caller -- to attract overseas buyers since the service started 13 months ago. AT&T said the telephone service costs Harrods $90 for each hour that the toll-free number actually is used by customers.

Starting Friday morning -- the day the Harrods advertisment announcing the new service appears in The New York Times -- and running until Jan. 5, customers in the United States can dial directly to an order desk at Harrods, in the Knightsbridge section of London. The purchases can be charged on an American Express card. Instead of refunding the British value-added tax, Harrods will pick up the cost of shipping the purchases to the United States by air.

The number, which will be answered around the clock, is 1-800-248-5555.

The sale, which traditionally attracts customers from around the world, actually runs from Jan. 8 -- three days after the toll-free telephone service for Americans ends -- to Feb. 1. An estimated 300,000 shoppers are expected to pour through the store, which bills itself as the world's largest, during the first two days of the three-week sale.

Last year's sale, held when the pound was worth $1.15, attracted unprecedented numbers of Americans who found the bargain prices made it worthwhile to fly to London. They were able to buy cashmere sweaters at $73.

The prices are higher this year, with the pound now standing at $1.425. Harrods said it will charge $85 for a lady's sweater and $130 for a man's sweater. A lady's cashmere coat costs $225.

"We are very pleased that Harrods can expand its annual January sale to include discriminating shoppers in the United States," said L. F. Drewitt, the store's managing director.

He said the firm expects the service " . . . will give us increased access to the American market."