The FTSE 100 and the mid-cap FTSE 250 were roughly flat by 0814 GMT.

Shell rallied for the seventh straight session, while BP was also a plus for the blue-chip index, as the potential for supply disruptions because of tensions between the United States and Iran pushed crude prices higher.

Investors will be watching intently on the outcome of President Donald Trump's upcoming meeting with Chinese counterpart Xi Jinping at the G20 summit in Japan, which could very well influence the course of a protracted trade war that has roiled the global economy.

Ahead of the meeting, Chinese Vice Commerce Minister Wang Shouwen said both countries should make compromises in trade talks, after negotiations broke down last month.

HSBC and Standard Chartered, both marginally down, kept the main index's gains in check.

Admiral Group was among the biggest gainers on the FTSE 100 after Barclays upgraded the insurer's rating by two notches.

"Our analysis suggests insurance prices have reached an inflection point in the second-quarter... we see this as a positive for Hastings and Admiral," Barclays analysts wrote.

Mid-cap insurer Hastings added 1.1%.

Relx, an information and analytics provider, rose   1.8% after Credit Suisse raised its price target on the stock.

Supermarket chains Tesco and Sainsbury slipped, however, giving up 2% each after a forecast showed consumer spending in Britain this year would grow at its slowest rate in six years.

On the mid-cap index, Dixons Carphone dropped 5% as last week's disappointing annual results prompted further rating downgrades of Britain's biggest seller of electricals and mobile phones.

Aston Martin skidded 1.5% as Mercedes-Benz owner Daimler's profit warning triggered a sell-off among European carmakers.