SIRTE, Libya (Reuters) - A close friend of Libyan leader Muammar Gaddafi who helped steer the OPEC member out of diplomatic isolation became head of the country's top legislative body on Tuesday.

Mohamed Abdul Quasim al-Zwai, who was Libya's first envoy to Britain after the two countries renewed diplomatic relations in 1999, replaced Moubarek Abdallah al Shamekh as secretary of the secretariat of the General People's Congress (GPC) after a vote.

The GPC names the government and is the top consultative body in Gaddafi's system of grass-roots government in which political parties are banned.

The secretariat, which was cut to seven members from 12 this week to streamline decision making, coordinates the GPC's agenda and deliberations.

Experts on Libya say that, in practice, no GPC resolutions can be passed without the leader's approval.

Zwai was a school classmate of Gaddafi and joined him in overthrowing the north African country's ailing King Idriss in 1969.

In 2001 he became Libya's first ambassador to Britain in 17 years and worked closely with Gaddafi's reformist son Saif al-Islam to convince the British and U.S. governments that Libya wanted to scrap banned weapons programmes and end its isolation.

Inside Libya, Zwai is viewed as a technocrat and relative moderate. "He's seen as a pragmatist," said one Western former diplomat on condition of anonymity. "He has a good reputation and great influence, with lots of ministerial background."

Libya watchers say Gaddafi prefers to keep his vision for the country vague and they monitor top appointments closely for clues to where the government is headed.

Local media reported in October that Saif al-Islam had been named coordinator of a group of tribal, political and business leaders, making him Libya's second most powerful figure, but analysts say he is competing for influence with a conservative old guard.