Ideally, he said, the craft would climb to a maximum altitude of about 14,000 feet and descend slowly over Croydon. But this flight plan, he said, would require it to move through several busy airline corridors, and some kind of dispensation from traffic controllers would be necessary.

The Solar Challenger will carry, like conventional airplanes, a ''transponder,'' which transmits a radio signal giving its position and altitude to radar controllers on the ground, he said.

But it was possible, he said, that the craft may be required to fly much lower than desired in certain controlled air space, such as that around Croydon, where air traffic controllers may impose a ceiling of 2,000 feet because of airline traffic.

Noting that less solar energy to propel the plane was available at lower altitudes because of atmospheric absorption, he said: ''The plane could fly at 5,500 feet, but it has more power at altitude, and the biggest question we have is: How low will we have to fly? We'll get as close to London as we can.''

He said he expected the plane to climb at a speed of 20 to 25 miles an hour - at sea level it gains about 150 feet of altitude a minute - and to travel in level flight at about 33 miles an hour at a cruising altitude in the range of 6,000 to 10,000 feet. Its top speed would be 41 miles an hour if it should reach the maximum anticipated altitude of 14,000 feet. Splashdown Is Not Planned

No rescue units will be waiting in the English Channel, as they did in the Gossamer Albatross's flight, in case the pilot has to ditch.

''The channel accounts for only 10 percent of the trip,'' he said. If clouds block the sun's rays, the Solar Challenger will be high enough to glide safely to English soil from any altitude higher than 5,500 feet and land in a field, as a glider might.