The main index added 0.8%, with a boost from Shell  and BP helping the index recover most of the previous session's losses. The mid-cap FTSE 250 gained 0.5% by 0757 GMT.

BAT and other stocks that book a major chunk of their earnings in dollars gained as the pound drifted lower again, ahead of Prime Minister Boris Johnson's meeting with German Chancellor Angela Merkel.

After the European Union rebuffed Johnson's demand to reopen the Brexit divorce deal on Tuesday, investors are looking for fresh signals that Europe would be willing to re-negotiate and help Britain stave off a disorderly exit from the bloc.

More broadly, traders have pinned their hopes on the U.S. Fed and other central banks lowering interest rates in the near-term to cushion blows from a bruising U.S.-China trade war and quell fears of an impending recession.

Such uncertainties have driven away investors from risky assets such as equities in recent weeks, putting the FTSE on course for its worst month in four years and making the Fed's minutes all the more crucial.

"There is likely to be interest as to whether there was any discussion about whether a cut was even necessary, or more importantly whether an even bigger cut of 50 bps was discussed," CMC Markets analyst Michael Hewson said.

"The tone of the discussions will also give a steer as to whether markets are right to expect another move in September."

Merging mid-cap lenders OneSavings and Charter Court Financial gave up 4.1% and 2.2% respectively, after they warned of Brexit impact on housing and credit.

Rating actions also drove steep moves among mid-cap and small-cap stocks.

Outsourcing group Capita surged 6.2% to the top of the FTSE 250 after Goldman Sachs upgraded the stock.

Small-cap Nostrum Oil & Gas plummeted 22% to an all-time low after Berenberg slapped the stock with a double-downgrade on its rating and slashed price target to 25 pence from 200 pence.