By Carla Mozee, MarketWatch

LONDON (MarketWatch)--BP PLC emerged as the best performer among U.K. blue-chip stocks Friday, a day after its shares suffered their worst session in more than four years following an unfavorable court ruling related to the 2010 Deepwater Horizon disaster.

FTSE 100: The index itself fell 0.3% to 6,855.10, weighed in part by a 2.2% drop in Royal Bank of Scotland as Berenberg said RBS is overvalued by 30%. "RBS has had two good quarters, but the core profitability of the franchise remains unconvincing," wrote analysts James Chappell and Eoin Mullany to clients. They kept their sell rating on the bank.

London Stock Exchange shares also fell 2.2% after Borse Dubai sold 8.5 million shares, or a 3.1% stake, in the exchange operator, according to news reports.

BP gains: BP (BP) shares finished up 2.6% in heavy volume in London that surged by more than twice the daily average of about 22.1 million shares.

BP plans an immediate appeal of a ruling by a U.S. federal judge who found the company grossly negligent in the 2010 Gulf of Mexico oil spill., the company said late Thursday. Citigroup upgraded BP to a buy rating from neutral after the shares dropped 5.9% on Thursday, saying BP will likely avoid the worst-case financial implications of the ruling. BP could face $18 billion in civil penalties in the U.S. BP has already paid close to $30 billion in claims related to the deadly rig explosion and oil spill.

U.S. District Judge Carl Barbier assigned 67% of the blame on BP, with 30% going to Transocean (RIG), and 3% on Halliburton (HAL) . "The one mitigating factor is that BP is held only 67% responsible, a lower percentage than we had previously assumed," said analysts at Barclays on Friday. It is "likely to be a number of years before any fines are paid," they said, adding they rate BP shares at equal weight.

Elsewhere in London trade, most mining stocks lagged behind, with precious-metals producer Fresnillo ending down 4.6%. Randgold Resources Ltd. fell 4.1%, their fourth consecutive loss. Earlier this week, Deutsche Bank downgraded the gold producer to a hold from buy rating.

The FTSE 100 posted a weekly gain of 0.5%.

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