By Sara Sjolin, MarketWatch

LONDON (MarketWatch) -- U.K. stocks made solid gains on Friday, with the FTSE 100 index headed for its best week since early May, after the Bank of Japan took investors by surprise and announced more stimulus.

The London benchmark climbed 0.8% to 6,514.16, setting it on track for a weekly gain of 1.9%. Such an advance would mark the best week since the week ended May 2.

The trading optimism followed an upbeat mood in Asia, where Japan's Nikkei 225 index surged 4.8% after the country's central bank voted to expand the pace of its quantitative easing. The yen (USDJPY) tanked on the news, with the dollar touching its highest level against the Japanese currency since 2008.

In the U.K., shares of Royal Bank of Scotland Group PLC (RBS) added 1.6% even after the lender said it will set aside GBP400 million ($640 million) to cover foreign-exchange probes, as it reported a rise in profit in the third quarter.

British Airways parent International Consolidated Airlines Group SA (ICAGY), picked up 2.8% after raising its full-year earnings guidance, with a turnaround at its Spanish unit Iberia spurring a 30% rise in operating profit in the third quarter.

On a more downbeat note, shares of Randgold Resources Ltd. dropped 2% as gold prices continued to slump.

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