LONDON (Reuters) - Britain's smaller listed companies can reverse a "doom loop", driven by plummeting valuations and higher costs and regulation, with help from broader tax exemptions and incentives, a think tank said on Thursday.

Smaller companies represent only 8% of the combined value of Britain's bourse. But they account for over 80% of listed UK companies, provide key jobs and investment and can become tomorrow's stockmarket darlings with help from the government, regulators and industry, New Financial said.

Two weeks before Finance Minister Rachel Reeves presents her first budget on Oct. 30, the think tank suggested extending exemptions from stamp duty, a tax on share trading, to include the smallest 250 listed firms along with differential rates on capital gains or dividends on British companies.

"Making recommendations on tax and incentives a few weeks before a budget ... is perhaps a foolish exercise.

"But there is an opportunity for a new government focused on growth to adjust incentives for investment in smaller companies," it said in an analysis on the decline in smaller listed companies.

Reeves, whose party came to power in July on a promise to get Britain's economy growing, has ruled out raising the rates of income tax, national insurance and value added tax, but has also said she has to fill a 22 billion pound ($29 billion) hole in the public finances.

Smaller listed companies have borne the brunt of a collapse in demand by institutional and retail investors. The number of smaller company market debuts plunged by 80% to just 22 over the past decade, while the value raised by new issues has dropped by 90%, New Financial said.

UK smaller company funds, meanwhile, have recorded a 36th consecutive month of outflows, while the number of local authority pension schemes with a dedicated allocation to the sector has fallen from 18 to just one over the past decade.

But Australia, Canada and Sweden, where smaller companies have been thriving, are offering hope. "A concerted effort by government, regulators and the industry can set smaller companies back on track," the think tank said.

($1 = 0.7683 pounds)