The FTSE 100 added 0.7% by 0811 GMT. The mid-cap FTSE 250 outperformed with a 1% rise, helped by a near 30% surge in Peppa Pig owner Entertainment One after it agreed to be bought by U.S. toy maker Hasbro.
Entertainment One shares hit a life high at 579 pence after a 560 pence a share deal with Hasbro. News of the deal lifted shares in broadcaster ITV by 4% to the top of the FTSE 100.
In the previous session, UK markets fell sharply as the Fed's latest minutes showed a deeply divided view on future monetary policy and a rally in the pound hit blue-chip exporter stocks.
Fed chief Jerome Powell's speech at 1400 GMT at the Jackson Hole Symposium is expected to provide a clearer picture on future rate cuts.
Traders took heart from a comment by White House economic adviser Larry Kudlow that the Trump administration is still planning for a round of in-person talks between U.S. and Chinese officials in September.
Fears of how the bruising U.S.-China trade dispute would affect the global economy and worries of an impending recession have put the main index on track for its worst month in four years.
"There has been so much made about Powell's appearance today that he's going to have to put on the performance of a lifetime just to avoid disappointing the crowd," said Craig Erlam, senior market analyst at Oanda.
Analysts at London Capital Group also played down expectations, saying Powell may probably not be in a position to give the markets what they want.
Meanwhile, sterling retreated from a three-week high after dealers took stock of Prime Minister Boris Johnson's meetings with German Chancellor Angela Merkel and French President Emmanuel Macron and their implications on Brexit.
As a result, companies that book a major chunk of their earnings in dollars, such as Diageo and Unilever , advanced.
Mid-cap Woodford Patient Capital Trust hit an all-time low after it said its total net asset value would be hit by a move by Link Fund to lower the value of the company's stake in IH Holdings. Woodford shares were last down 5.6%.