By Barbara Kollmeyer, MarketWatch

MADRID (MarketWatch) -- The U.K.'s FTSE 100 index moved slightly higher on Wednesday, supported by gains for mining companies, as investors focused on central banks and tried to stay calm ahead of a nearing referendum on Scottish independence.

London's benchmark index rose 0.3% to 6,810.05, but still lagged gains for bourses across Europe. The index has been down for two straight sessions, amid investor caution over the potential for a "yes" outcome to the vote to leave the U.K. in Scotland, where polls open Thursday. Read: Good riddance, Scotland: We don't need you

The pound (GBPUSD), which has been suffering from uncertainty surrounding the vote, showed some strength, trading at $1.6310 versus $1.6282 late Tuesday.

On the U.K. economic calendar, unemployment data and the minutes of the latest Bank of England Monetary Policy Committee meeting are due early. Investors will be looking to see if any more members voted to hike rates, as the August meeting showed the first dissent among members in several years. Both the joblessness data and BOE minutes are due at 9:30 a.m. London time.

The two-day Federal Open Market Committee meeting could keep investors at bay. The focus will fall on the Federal Reserve policy statement and any change in language that could hint of a sooner-than-expected hike in interest rates. Read: The only two words that will matter at the Fed

Stocks:Smiths Group PLC topped the losers list with a nearly 5% drop after the medical-device and airport-scanner maker posted a fall in full-year revenue and profit, dragged by a weak performance at its detection business.

Miners were on the rise, with heavyweights such as Rio Tinto PLC (RIO) up 0.7% and BHP Billiton PLC (BHP) adding 0.6%. Anglo American PLC rose more than 1%.

Lloyds Banking Group PLC (LYG) was a heavyweight gainer, up nearly 1%, and chip designer ARM Holdings PLC (ARMHY) added 1.8%.

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