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* FTSE 100 down 0.6 pct
* FTSE 250 down 0.7 pct
* Miners, exporter shares drag on main index
* Metro Bank hits life-low after Q1 results
May 2 (Reuters) - A fall in blue-chip copper miners and a firmer sterling drove the FTSE 100 lower on Thursday, while mid-cap Metro Bank tanked after an accounting error caused quarterly profit to halve.
The FTSE 100 .FTSE was down 0.5 percent and the FTSE 250 .FTMC was 0.7 percent lower by 0730 GMT.
Sterling gained after investors took comments made by Prime Minister Theresa May as an indication of progress in Brexit talks between the government and the main opposition Labour Party. led to shares of dollar earners such as British American Tobacco BATS.L and Diageo DGE.L shedding more than 1 percent.
Copper prices hit a more than two-month low, causing an index of miners .FTNMX1770 to fall for the seventh consecutive session and leading heavyweights Rio Tinto RIO.L , Glencore GLEN.L and BHP BHPB.L more than 2.2 percent lower.
But medical products maker Smith & Nephew SN.L outperformed the main bourse with a 2.9 percent gain, as it forecast annual revenue growth at the top half of its prior guidance range. the mid-cap index, Metro Bank MTRO.L slumped 16 percent to an all-time low after saying the fallout from an accounting error it disclosed in January hit quarterly profit, capital buffers and major business customer deposits.