The FTSE 100 held steady as investors remained cautious in light of a new U.S.-EU trade tussle and before a Brexit summit, while a U.S. indictment wiped out two-thirds of drugmaker Indivior's value and dragged down former parent Reckitt Benckiser.
The FTSE 100 was marginally lower while the FTSE 250 was up 0.1 percent by 0839 GMT.
Markets were largely subdued due to a transatlantic aircraft subsidy dispute, with the U.S. threatening to impose tariffs on $11 billion worth of European Union products.
"The market is obviously taking a cautious look at the simmering EU-U.S. trade spat and preparing for a negative outcome but hoping for a happy ending," Markets.com analyst Neil Wilson said.
However, the pound firmed after data showed Britain's economy unexpectedly grew in February, helped by clients of manufacturing companies rushing to stockpile goods in the run up to Brexit.
To that end, market participants opted for a wait and watch approach ahead of a Brexit summit in which EU leaders are likely to grant British Prime Minister Theresa May a second delay to Britain's exit from the bloc, but could demand an extension beyond June 30.
The most notable moves came after the U.S. Justice Department accused Indivior of illegally boosting prescriptions for the film version of its blockbuster opioid addiction treatment Suboxone.
As a result, Reckitt slipped 5.6 percent on the main index and Indivior lost more than two-third of its value on the mid-caps. Its shares, which have been pummelled by a protracted legal battle with India's Dr. Reddy's, were last seen down 70 percent.
Jefferies analysts said Indivior could require a major rights issue to raise enough money to pay any prospective fine.
Retailers Tesco Dunelm and ASOS were among bright spots on the market after their respective financial reports.
Tesco, Britain's biggest retailer, rose as much as 2.2 percent after a better-than-expected rise in full-year operating profit, while mid-cap Dunelm added 2.7 percent as higher quarterly comparable sales helped the homewares retailer forecast full-year profit to top analysts view.
AIM-listed ASOS, whose profit warning had triggered a global retail selloff in December, jumped 6.3 percent as investors were comforted by the fashion retailer's affirmation of annual targets despite a plunge in half-year earnings.