Former prime minister Ian Smith, after a six-day-long personal effort aimed at sounding out white public opinion here, flew back to London today, apparently softening his opposition to Britain's draft constitution proposal.

At a press conference before his departure to rejoin the London talks on Rhodesia, Smith said he may ultimately have to accept British proposals to eliminate the white minority's blocking veto on legislation and white control over certain key government appointments.

Earlier today, Salisbury's electricity system was partially cut by what government officials termed a "disruption" of transmission lines by antigovernment guerrillas. It was the fifth such guerrilla attack in recent weeks.

In neighboring Zambia, Zambian officials said today that forces of Zimbabwe-Rhodesia Prime Minister Abel Muzorewa have blown up two bridges to cut off Zambia's rail and road links with neighboring Tanzania. The disruption of the Tazara railway line leaves Zambia totally dependent for its exports on a rail line through Zimbabwe-Rhodesia and South Africa.

Smith, who is now minister without portfolio in the Muzorewa Cabinet asserted last Sunday that the constitution proposed by Britain would be rejected by the biracial Parliament here despite its acceptance by the prime minister and 11 other members of his delegation at the London talks.

Today, Smith reaffirmed that he did not like the constitutional proposals, but added, "In the end I may have to accept even things I don't like and that's what happens when you come to the final package."

By "final package" Smith is referring to proposals the British governement will submit to the Muzorewa delegation and their guerrilla opponents in the Patriotic Front for the overseeing of new elections and in particular their suggestions for resolving the ticklish problem of who will control the armed forces during the transitional period and in a new governement thereafter.

It appears that Smith's more restrained attitude today is a consequence of a meeting he had with the parliamentary caucus of his Rhodesian Front Party last Monday during which he reportedly came under some criticism for speaking out so harshly against the British proposals.

Whites here are concerned that they not appear to be the recalcitrant party in the London talks because they are hoping for a settlement that will include the lifting of economic sanctions against Zimbabwe-Rhodesia and diplomatic recognition from Britain and the United States.

After the meeting, the caucus issued a statement rejecting the adoption of "hard and fast attitudes" at this stage until the "final package" was known.

Smith's more placatory attitude may also be due to South African pressure. Shortly after Smith's statements, South African Foreign Minister Pik Botha urged the Muzorewa delegation not to discuss its differences in public. It is known that the South African government does not view with pleasure Smith's present public stance.