A Pennsylvania college professor and his stockbroker friend embarked five years ago on a "Swiss hit-or-miss proposition" and turned it into such a successful travel operation that they gave rebates to clients last year and lowered their prices this year.

They have now expanded the operation to London. Called the Britsih Untour, the new program -- like its precursor, the Swiss Untour -- provides fully-furnished, family-sized apartments at a fraction of what comparable accommodations would cost at a hotel. In addition, Untour participants get a number of important support services, including advice on shopping for food, reading train timetables, using public transportation and planning excursions.

"The concept of our Untours," says Harold E. Taussig, adjunct professor of American Studies at Pennsylvania State University, "is to give the traveler an alternative to tightly scheduled package tours without the insecurities of do-it-yourself travel. Untourists can experience Switzerland and Britain on their own terms, pursuing events and activities determined by their interests only, and they will return home having discovered a new land and having claimed it as their own."

Taussig, who still teaches, began his new career in travel quite by accident in the early 1970s after he had returned from a sabbatical in Europe -- mostly in Switzerland, where his son and daughter-in-law live. He wrote a book about his experiences, "Shoestring Sabbatical," which is out of print, and inquiries from fellow teachers began to pour in. In 1976, he approached a friend, Walter Baker, a Philadelphia stockbroker, with the idea of setting up apartment rentals in Switzerland for teachers on sabbatical.

The two of them set up a company "Idyll Ltd.," and went to Switzerland to search out apartments in chalets between Lucerne and Interlaken, "a startingly beautiful lake-mountain area."

During the past 10 years, as the value of the dollar declined sharply overseas and American visitors began to avoid Switzerland because it was too expensive, the number of Americans that Idyll took there more than double. "Last year," said Taussig, "our business was up 20 percent over 1079 and this year advance bookings point to a 200-percent jump over 1980."

In fact, said Taussig, gusiness was so good last year that participating Untourists received a rebate averaging $40 for their rental unit. "by increasing our numbers, we passed that breakpoint which produces a reduction in overhead. It's our philosophy that business should pass such economies on to the consumer."

Prices for this year's Swiss rentals are about $10 less than last year, Taussig said.

"When figured on a per-person basis, our 1981 prices range between $8 and $17.50 a day -- and that includes a lot more than a place to stay. With hotel rooms running more than double that, more and more people are interested in what we offer."

The Idyll fee this year for a three-week term is $735 for two people, $840 for three, $930 for four, $1,015 for five, $1,075 for six and $1,150 for seven. Taussig, who is also president of Idyll, said the price includes support services such as an escort from the Zurich airport to the apartment, individual orientation sessions to help each person plan his Untour, and newsletters featuring advice on how to get behind the scenes in Switzerland. The Swiss Untour season runs from May 14 through Oct. 9.

The reason Taussig and company are invading the British market this year, he said, "is because of popular demand by our Swiss Untour alumni. We had a poll asking them where they would like to see our concept repeated, and Britain won hands down."

Unlike the Swiss program, where the apartments are located in villages and towns outside of major cities, the British Untour features accommodations in London.

According to Taussig, all the apartments are located in Victorian townhouses in the residential Victoria-Pimlico and Bayswater sections of the city. "The apartments are close to all the city's attractions as well as the British Rail network. Nearby are all the shops needed to fill the apartment's larders, and restaurants, open-air markets and parks."

The overall charge for a basic two-week term -- the season runs from June 1 through Labor Day -- for a studio apartment, double occupancy, is $1,090, which, according to Taussig, comes out to $38.90 per person per day. If six people share a two-bedroom apartment, the two-week price is $2,150, which breaks down to $25,60 per day per person, he said. "Untourists can really save money by using their kitchen," Taussig said. "A family of four, for instance, could save up to $700 over two weeks by cooking in their own kitchen and avoiding restaurants."

Included in the basic price are all the amenities offered in the Swiss Untour -- escort service from the airport, orientation meeting and advice on food shopping, getting around and touring. In addition, Taussig said, the British Untour features daily maid service.

You can get information on both the British and Swiss Untour programs by writing to Harold Taussig, Idyll Ltd., Post Office Box 405, Media, Pa. 19063.