* FTSE 100 index up 0.1 pct
* Briefly hits 14-1/2 year high after ECB unveils new stimulus
* BP falls; U.S. court says firm 'grossly negligent' in 2010 spill
* Standard Life surges on deal to sell Canadian assets
By Tricia Wright
LONDON, Sept 4 (Reuters) - Britain's top shares rose on Thursday after the European Central Bank cut interest rates and announced plans to buy assets in a bid to shore up euro zone inflation, though steep falls in BP capped gains.
BP shares fell 5.9 percent, suffering their biggest one-day percentage drop since June 2010, after a judge in the United States said the oil major had been "grossly negligent" for its role in the 2010 spill in the Gulf of Mexico.
The share price drop wiped some $8.7 billion off the company's market capitalisation. The ruling could add billions of dollars in fines to the more than $42 billion in charges taken so far for the worst offshore disaster in U.S. history.
BP said it would appeal the court decision.
Traders took the view that the share price drop would prove relatively short-lived.
"Obviously the market's not taken it well and it was a little bit unexpected ... (but) it is a short-term concern; longer term BP are cash generative and I'm sure they'll have the funds to pay for this," TJM Partners' head of trading, Manoj Ladwa, said.
Trading volume in the stock stood at more than four times its 90-day daily average, against 113 percent on the broader FTSE 100.
The index, which briefly hit a 14-1/2 year high at 6,904.86, closed up 4.39 points, or 0.1 percent, at 6,877.97.
The market was given a fillip earlier in the session when the ECB unexpectedly cut interest rates to new record lows and the bank's president Mario Draghi said it would start buying securitised loans and covered bonds next month to help unblock lending in the euro zone.
But with the FTSE 100 up some 5 percent since early August, bolstered by expectations of the new economic stimulus measures, some traders reckoned the rally has run its course.
"It was factored in - it's too little too late as far as the market's concerned," TJM Partners' Ladwa said.
Other sharp movers included Standard Life, which surged 8.1 percent in brisk trade after agreeing to sell its Canadian operations.
The UK life insurance index climbed 1 percent following Standard Life's deal worth about C$4 billion ($3.7 billion) in cash with Manulife Financial Corp.
Trading volume in Standard Life stood at almost seven times its 90-day daily average.
The companies said they will also expand an existing wealth and asset management partnership, with Manulife distributing Standard Life funds in Canada, the United States and Asia, and Standard Life doing the same in the UK retail market.
"(This) accelerates Standard Life's strategy of asset gathering and asset management and removes a major exposure of the group's balance sheet to spread and guarantee risk," Eamonn Flanagan, analyst at Shore Capital, said.
"The agreement for Manulife to act as distributor for SL's funds should deliver growth in funds, thus accelerating Standard Life's focus as an asset gatherer and manager."
The Bank of England also held a policy meeting on Thursday. It kept interest rates on hold. (Additional reporting by Atul Prakash; Editing by Susan Fenton)