Britain's FTSE falls back with Ashtead, commodities under pressure

* FTSE 100 index falls 0.9 pct

* Ashtead down on read-across from United Rentals (NYSE: URI - news)

* Centrica (LSE: CNA.L - news) , Aberdeen Asset Management (Other OTC: ABDNF - news) fall on downgrades

* Anglo American (LSE: AAL.L - news) rises after production update (Recasts, adds detail and quote, updates prices)

By Kit Rees and Atul Prakash

LONDON, Jan 28 (Reuters) - Britain's top stock index retreated after hitting a three-week high earlier in Thursday's session, led lower by Ashtead and commodity-related stocks.

Equipment rentals company Ashtead was the top faller on the blue-chip FTSE 100 index, down over 6.5 percent on a read-across from U.S (Other OTC: UBGXF - news) . peer United Rentals, which reported an underwhelming set of fourth-quarter results.

"Even (Taiwan OTC: 6436.TWO - news) though Ashtead are listed over here, they are predominantly a U.S.-exposed company," Manoj Ladwa, head of trading at TJM Partners, said, adding that a slowdown in the U.S. market wasn't helping the stock.

The blue-chip FTSE 100 index was down 0.9 percent at 6,939.63 points by 1226 GMT after earlier rising over 6,000.00, its highest since early January.

The benchmark index is still down nearly 5 percent following a commodities-led sell off earlier this year.

Also among the fallers was energy supplier Centrica, down 4.9 percent following a downgrade to "sell" by Societe Generale.

Aberdeen Asset Management fell 2.4 percent on a spate of broker downgrades after indicating on Wednesday that more clients pulled cash from its funds in the December quarter due to concerns about economic growth.

After rallying earlier in the session, commodity-related stocks trimmed their gains as the price of oil inched lower and the price of copper fell ahead of China's New Year holiday, when activity in the world's biggest consumer of copper and iron ore is expected to slow.

Shares (Berlin: DI6.BE - news) in mining companies BHP Billiton (NYSE: BBL - news) , Rio Tinto (LSE: RIO.L - news) and Glencore (Xetra: A1JAGV - news) turned lower to trade between 0.1 to 2 percent down.

Shares in Anglo American, however, jumped over 9 percent after the company said it produced more iron ore last year. Iron ore is one of the biggest earners for Anglo American, which also produces coal, copper, platinum and diamonds.

The UK Oil and Gas index turned to trade slightly lower, up just 0.2 percent after the price of oil retreated slightly.

The mood remained fragile after the U.S. Federal Reserve said on Wednesday it was "closely monitoring" global economic and financial developments, signaling it had accounted for a stock market selloff but wasn't ready to abandon a plan to tighten monetary policy this year.

Data showing that Britain's economy ended 2015 on a soft note also didn't aid sentiment. The annual pace of growth slowed to its weakest in nearly three years.

Among mid-caps, British transport company FirstGroup (Amsterdam: FI6.AS - news) dropped more than 9 percent after it warned its annual profit would come in lower than it expected, citing wet weather and flooding in Britain and driver shortages in the United States. (Editing by Hugh Lawson)