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* FTSE 100 up 0.3 pct
* Centrica, SSE dip as investors eye Conservative campaign vow
* Micro Focus sinks on as buyout target's revenue falls
* Mining shares underpin gains; utilities fall back
* Asset manager Henderson gains as UBS ups to 'buy'
By Helen Reid
May 9 (Reuters) - Energy companies Centrica and SSE were among the worst-performing stocks on Britain's FTSE on Tuesday, after British Prime Minister Theresa May vowed to cap energy prices if she was re-elected in June.
While the blue-chip index was up 0.4 percent at its highest level since early April, big declines stole the limelight.
Prime Minister May pledged to introduce a ceiling on domestic energy prices that would cut tarriffs for around 17 million families. She said energy regulator Ofgem would be ordered to cap the costs of standard variable tariffs, the package used by two-thirds of customers in Britain.
Centrica fell 3.6 percent and SSE 1.8 percent, continuing a decline that began when the Conservatives first hinted at the policy, which could erode their margins.
"The government has stated that two-thirds of customers are on the higher 'standard' rates, and clearly if the cap is below this level this will hit margins directly," said Edward Park, director of the investment committee at Brooks Macdonald.
"But the government will need to be careful not to cause excessive pressure on the energy market as capex in areas such as 'smart meters' are also viewed as helping retail customers make informed decisions," he added.
The cap could cost Centrica up to 200 million pounds, Neil Wilson of ETX Capital estimated.
Micro Focus sank as much as 10 percent to the bottom of the FTSE after it flagged a 10 percent revenue drop at Hewlett Packard Enterprise, the company it's in the process of acquiring for $8.8 billion.
Mining companies Glencore, Antofagasta, Fresnillo and BHP Billiton gained the most, rising 2 to 3 percent as the price of copper edged up from sharp overnight losses.
Ferrexpo and Kaz Minerals led gains on the mid-caps as well, up 5.3 and 4.1 percent respectively.
Asset manager Henderson benefited from a UBS upgrade and rose to join the top mid-cap gainers, up 3 percent and on track for its best day in six months. UBS analysts see its acquisition of U.S. fund manager Janus as "transformational" for the firm.
"Significantly enhanced scale, distribution and diversification see it better equipped to deal with ongoing headwinds from a gradual global shift to passive and rising regulatory costs," they said in a note.
Meanwhile the broker's downgrade of Card Factory to "neutral" after the shares rallied weighed on the stock, down 4 percent and the biggest mid-cap decline. (Reporting by Helen Reid, editing by Larry King)