IT WAS LOVE at first sight. So I told three friends, and they told three friends, and they told three friends. And now standby seats on British Caledonian's new London-Hong Kong flights are as popular as free samples from Ft. Knox.

Well, I guess we're not solely responsible. At roughly $240, that's the sort of ticket offer that inspires travelers to rise right out of their armchairs.

But do look now, because there are more peaches on the same tree. In fact, some industry observers figure "standbys" -- or something close to them -- will be the biggest and therefore best deals in international bargain fares this winter, though it's impossible to say in terms of actual seat numbers.

Trade magazine publisher Clif Cooke, for one, expects to see some kind of standby plan from every U.S. gateway city -- in other words, a chance for flexible flyers who are on the scene and can live with the uncertainty of last-minute arrangements to grab leftover seats at genuinely likable prices. His forecast already looks like a winner for anyone thinking of Europe or the Far East. For instance:

There are various versions of standby seats to London available from Anchorage, Atlanta, Baltimore, Boston, Chicago, Dallas, Detroit, Honolulu, Houston, Los Angeles, Miami, New York, Oakland, Philadelphia, St. Louis, San Francisco, Seattle and Washington, D.C. The airlines are Air India, Braniff, British Airways, British Caledonian, El Al, Laker (Skytrain), Pan American, TWA and World -- one or more from each of the named cities.

In addition, though, there's a smorgasbord of other possibilities, including Amsterdam, Brussels, Frankfurt and Paris via Boston and Dallas with Braniff (until Oct. 15); Vienna from New York with Montana and Tarom; Brussels and Frankfurt from New York with Capitol; Prestwick from Boston, New York and Seattle with Northwest Orient; Glasgow and Manchester from New York with British Airways; Bangkok from Los Angeles and Seattle with Thai Airways; Manila from San Francisco with Philippine Airlines.

Within this country, a standby variation with sales of leftover tickets the day before each flight makes big savings possible between a long list of city pairs in the Northwest. Hughes Airwest started the ball rolling, and its competitors have kept it up.

If you're now expecting me to give the prices for all these places, give up.

I am not so foolish. The examples mentioned could themselves double or disappear within the next half hour, and prices, well -- who known?

Okay, I will go out on a limb and mention a few figures that just might hang around for a while. Capitol has some real grabbers: $99 to go between New York and Los Angeles; $150 between New York and Brussels; $169 New York-Frankfurt. Tarom, which you might have figured was either a misprint or a mystery carrier, is actually the national airline of Romania and no, you can't go standby all the way to Bucharest, just Vienna. It's $180, and there are once-a-week flights leaving both directions on Wednesdays. Montana, incidentally, refers not to the state but to a private Austrian airline.)

Then there's Pan Am with Honolulu from Los Angeles and San Francisco for $138; between New York and Detroit, $58; between San Francisco and Seattle, $63. Honolulu to Los Angeles (and that's direction only; not Los Angeles to Honolulu) is $119.99 standby with World. Thai from Los Angeles and Seattle to Bangkok is $380. Oh, yes: Pan Am is still maintaining a standby price of $1,199 onits around-the-world ticket.

Having gone this far,I might as well go farther and make five fearless predictions. To wit:

1. Sir Freddie Laker will turn permanently polka-dotted before he is undersold on any of his London routes. And whatever his newest, newly-revised fall-winter Skytrain fares turn out to be, his no-reservations, first-come-first-served version should be the closest to rock-bottom that fares to the British capital are likely to get this or next year.

2. The Astonishing Achievement Award will go to Continental Airlines for creating a standby fare between Ft. Lauderdale and Miami. There might be those who think they could jog this distance faster, but never mind.

3. World will corner the impulsive lovers market. Well, who else but wild and crazy couples are going to buy first-class standby seats to London for an extra almost 200 greenies per seat? You can try to drink that up on board in free French champagne, but chances are your insides won't let you.

4. Despite this warning, some bargain hunters will get it into their heads that nothing but nothing is cheaper than standby. They will also have fainting fits when they discover too late there are exceptions.

Northwest Orient, for instance, has some Budget Fares for confirmed seating (with a wide variety of purchase restrictions, to be sure) that offer very competitive rates to various destinations in Europe and the Far East. Singapore Airlines to the Orient does, too -- and throws in some excellent free stopover possibilities.

Note, too, that in this country Texas International might forever stunt the development of standby fares with their newest cheapie: 66 to 83 percent discount to passengers who sign up at least 14 days ahead for non-changeable, non-refundable tickets. There's a limit on the number of tickets offered under this setup, and not all of TI's cities are included, but the savings are whoppers. Example: $59 between Baltimore-Washington and New Orleans compared to "regular" fares to $190.

5. Despite this warning, some travelers will wind up in the soup by failing to grasp that every little airline has a meaning all its own for standby. Ask questions!

For British Caledonian, for instance, you mainly "stand by" the telephone.

In other words, call anytime after 9 a.m. the day before a flight and if they have a spare seat they'll confirm you then and there.

Some carriers require you get a queue number at a city ticket office before coming to the airport. Others have other rules, including paying for a ticket in advance or even going to the airport. The only thing wrong with that one is that most neglect to mention that if you don't get a seat and therefore want a refund, the only way to be sure you'll get it instantly is to pay in cash, not with a check or credit card. Nor should you assume that one airline's ticket can be "endorsed" for use on another. Some definitely can't, and some claim the right to choose.

Standby clearly isn't for everyone, but it's become a good way for airlines to sell spare seats -- and it looks as if they'll have a lot of those this winter.