The blue-chip FTSE 100 <.FTSE> index ended the day flat, mimicking a broadly flat European market.

Higher oil prices pushed up the commodities-heavy small and mid-cap indexes. The FTSE 250 <.FTMC> set a record high at 19,277.99 points, closing 0.2 percent higher at 19,264.60 points. Tullow Oil (>> Tullow Oil plc) was the top gainer, up 5.7 percent.

The small caps <.FTSC> also climbed to new heights, closing 0.4 percent higher, near its record high of 5,448.60 points.

Retailers ABF (>> Associated British Foods plc), Marks & Spencer (>> Marks and Spencer Group Plc), Tesco (>> Tesco PLC) and Next (>> NEXT plc) all gained. A court approved a deal between Britain's biggest retailer Tesco and the Serious Fraud Office to settle an investigation over a 2014 accounting fraud.

Shares in BHP Billiton (>> BHP Billiton plc) closed a strong session up 2.2 percent, after hedge fund Elliott Advisors sent a letter to the miner proposing a plan to unlock shareholder value. The plan involves scrapping BHP's London Stock Exchange listing, demerging its U.S. oil arm and revising its capital return policy.

"Being based in Australia, you've got a lot of resources in the country itself, you've got access to Asia which is a great consumer of those kinds of resources, but then the capital, historically, has always come from London - so that's essentially why you've got two listings," said Ken Odeluga, market analyst at City Index.

"I don't really see how much value would be unlocked by such a drastic action so quickly."

Precious metals miners Randgold Resources (>> Randgold Resources Limited) and Fresnillo (>> Fresnillo Plc) were the biggest fallers, down 2.4 and 2.5 percent respectively as the price of gold inched lower.[GOL]

Small-cap platinum miner Lonmin (>> Lonmin Plc) dropped 10 percent, tracking platinum prices lower.

Shares in Barclays (>> Barclays PLC) reversed earlier losses to trade 0.4 percent higher after the bank said British regulators were investigating its chief executive, Jes Staley, over the handling of a whistleblowing incident.

The investigation by the Financial Conduct Authority and the Prudential Regulation Authority relates to an attempt by Staley last year to identify the author of a letter by a whistleblower, the bank said.

"While Mr Staley's reputation has undoubtedly taken a serious knock, we believe that it remains in the best interests of shareholders to keep him in the post of CEO and hence we recommend that they follow the board's direction and vote in favour of his reappointment at the AGM in May," Gary Greenwood, an analyst at Shore Capital Markets, said in a note.

Shares in UK Real Estate Investment Trust Shaftesbury (>> Shaftesbury plc) rose 3.3 percent on a media report that a Hong Kong-based billionaire was considering a bid for the company.

(Reporting by Kit Rees, editing by Larry King)

By Kit Rees and Helen Reid