The top share index was up by midday on Friday, off a seven-week low earlier this week, after the UK's second-quarter gross domestic product was unexpectedly revised upwards and ahead of a speech from U.S. Federal Reserve Chairman Ben Bernanke.

By 11:41 a.m., the FTSE 100 <.FTSE> was up 2.94 points, or 0.1 percent, at 5,158.78, having closed up 0.9 percent at 5,155.84 in the previous session, rallying from a seven-week closing low on Wednesday.

Britain's economy grew faster than originally estimated in the second quarter of this year, reaching its fastest rate of growth in more than nine years, official data showed on Friday.

The Office for National Statistics said the economy grew by 1.2 percent in the three months to June, up from its preliminary estimate of 1.1 percent, after construction output proved stronger than first thought. Analysts had expected no revision.

What we continue to see is nervousness revolving around the potential for a weaker growth profile hitting investor sentiment and eating away at investors' risk appetite on one side, Henk Potts, equity strategist at Barclays Wealth, said.

But on the other side you've still got demonstrations of a relatively strong corporate picture. I suppose when you get those two concepts meeting, inevitably you'll result in an element of volatility.

Tullow Oil was the top faller, down 4.0 percent, after two Ugandan newspapers said the Ugandan government had refused to extend one of the UK company's exploration licence. Tullow declined to comment.

Other energy stocks <.FTNMX0530> fell in tandem with crude prices, paring the previous session's gains, with BP continuing to ebb and flow, down 2.0 percent.

Banks were also weaker as risk appetite deserted investors, with Lloyds Banking Group the top faller in the sector, down 1.2 percent.

EYE ON JACKSON HOLE

Fed Chairman Bernanke is scheduled to give a keynote speech later on Friday at a conference of central bankers from around the world in Jackson Hole, Wyoming.

In the speech, he is likely to signal his views about the uncertain prospects for the world's biggest economy but probably will not offer many clues on whether the U.S. central bank will pump in more cash to keep the recovery going.

Rolls-Royce fell 1.4 percent after it pushed back the delivery date of a test engine for Boeing Co's 787 Dreamliner, causing the U.S. planemaker to delay the first delivery of its new plane.

Thin seasonal trade accentuated many of the share price moves on the FTSE 100, as Cable & Wireless Worldwide climbed 3.6 percent.

We're just seeing light volumes ... But all eyes on the States and Bernanke later on this afternoon, Giles Watts, head of equities at City Index, said.

National Grid rose 1.0 percent with traders citing a BofA Merrill Lynch note, in which the broker resumed its coverage on Britain's biggest utility with a buy rating.

(Additional reporting by Tricia Wright; editing by Karen Foster)

United kingdom Ben Bernanke Uganda Ftse