What Jane Fonda has done for the "thunder thighs" of America, two English women are now doing for the "baggy bums" and "Guinness guts" of Britain.

Best known by their nicknames--"The Green Goddess" and "Mad Lizzie"--these "keep-fit ladies" present brief, follow-along workouts on England's new morning television shows. In the six months since "breakfast telly" began in Britain, millions of people have started launching their day exercising along with Diana Moran of the BBC's "Breakfast Time" and Lizzie Webb of ITV's "Good Morning, Britain."

"The keep-fit movement is at its peak here," says Moran, 44, a statuesque blond ex-model whose Green Goddess nickname comes from her trademark emerald leotard and tights. Prompted by "the American fitness movement," "the new knowledge about the relationship of disease to diet and exercise" and "the enormous popularity of the TV show 'Fame,' " she says, "people who would never have dreamed of exercising before have now gotten caught up in it."

As the glamor goddess of Britain's most successful morning show ("Breakfast Time" reaches as many as 8 million viewers during the 2 1/2-hour broadcast), Moran travels all over the country to tape her four-minute keep-fit segments.

"I might be with mums and their little babies on the beach at Brighton," she says, "or I'll be at Waterloo Station working out with the businessmen getting off the trains, or I might be sitting in a field in Jersey with the cows early in the morning.

"I like to work with people because it motivates those at home to slip off their shoes and join in. I want them to realize that keeping fit doesn't have to be a great, daunting, hour-long program. Whatever their age or shape they can stretch for three minutes and feel better.

"Then, perhaps, they can work on a target area--usually the tummy and hips for women and beer gut for men. Afterwards, they may be inspired to do a little bit more."

Moran's attitude illustrates the difference, she says, between the American and the British approach to fitness. "I don't believe in this business of requiring expensive equipment and clothes and shoes or a gym membership. I think you should take advantage of the here and now, whatever time you have, wherever you are--in the car, at home, standing in a queue--to tone and stretch."

The ideal exercise environment, she says, "is in your own home, in the nuddy, so you feel totally free." Second best: "In the fresh air with loose clothing."

The best exercise: "A great stretch for the whole body see box, above . Also, if you can hang from something--a tree branch, a door frame--you can let gravity do the stretching for you." Another excellent--but underappreciated--exercise: "Gardening. Pulling and bending and stretching in the open air is a wonderful, natural way to keep fit."

Moran also offers her audience nutrition advice. "British people particularly need to cut down on fat, since we're all reared on marvelously rich cream and butter." The traditional English breakfast of sausage and eggs is waning slightly in favor of whole-grain cereals, she says, as the result of new nutrition knowledge and the increase in "working mums."

Tea time is changing, too: "Slimmers are choosing lemon tea over milk tea and eating fresh fruit instead of scones."

Moran's fitness philosophy "just evolved," she says, "from my own explorations. I'm typical of anybody. As a youngster I was very sporty, then I got married at 19 and along came the children. One wasn't conscious, then, of keeping fit at all.

"But 12 years ago I was unwell with quite a serious throat operation. Suddenly I lay there and thought, 'You just don't stay healthy as a matter of course. You must look after yourself.' So I began taking yoga and exploring other forms of keeping fit."

About eight years ago she started teaching exercise at British "holiday camps"--large family vacation resorts.

"I would be in a huge ballroom with 200 people on the floor following me and another 400 sitting and watching. Over the course of an hour we'd do some stretching. I'd let them cool off while I talked about diet or had a little jolly, and then we'd start off again. That's where I get my 'C'mon, slip off your shoes and join in' approach."

When the American revolution in fitness first hit Britain about four years ago, she began hosting a weekly afternoon exercise program on one of England's independent TV stations. There she was spotted by producers creating "Breakfast Time."

Along with physical fitness, Moran promotes a philosophy of mental health: "Be open-minded, accept other people for what they are, keep a very strong sense of humor about life generally, don't get too precious and go out to meet people, as opposed to waiting for people to come to you."

On the opposite side of the TV dial and the personality spectrum is "Mad Lizzie" Webb, whose daffy, homey personality is in deliberate contrast to "Lady Di" Moran's regal bearing.

"Exercise has got to be fun," says Webb, 34, who holds degrees in English and dance education and has been choreographing and teaching dance for 12 years. "If it's not enjoyable, people will quit. My motto is 'Are You Doing It?' I want people to move, not just watch."

This is one reason why Webb always wears loose, comfortable clothes--rather than leotards--on the air. "I want people to view me as a sister or a mother, friendly and non-threatening. And I never talk about diet. I think we've gone too far with slimming. I think it's better to be stout and toned up than painfully thin and shapeless. I'd like fat people to feel comfortable about getting up and following along, too."

Webb models her five-minute in-studio segments after the "funky disco" dance classes she teaches at London's popular Pineapple Dance Studio in Covent Garden, where she got her nickname. "I'm always making jokes in class to keep the students motivated. One of my students worked for TV-AM, and when they started looking for a fitness person she mentioned 'This mad teacher I have.' "

She generally picks a theme for a week's work-out or relates exercise to people's daily lives. "I might show exercises you can do while you're cleaning the house. Or I'll talk about how to bend and lift without hurting your back since something like 60,000 people in the U.K. are off every day with backache."

While Webb is enthusiastic about the "enormous new interest in dance and exercise," she is "not too keen" on the push for aerobic dance classes. "They're fine if you're fit, and the teacher is good. But physios in London are picking up the injuries from classes with unqualified teachers who shout at you to 'Work, work, work until it burns.'

"I feel that if something hurts, something's wrong. Novice exercisers who stumble into that torture trap are going to be put off exercise for the rest of their lives."

In contrast, Webb's on-air exercises are "gentle. I always ask myself 'Is this something my mother could do?' " CAPTION: Picture, Diana "Green Goddess" Moran. Copyright (c) 1983, BBC