Shares of Shell and BP were by far the biggest drags on the blue-chip index, pulling the FTSE 100 0.2% lower by 0759 GMT even as global oil prices recovered some ground in morning trade in Europe.
Chemicals group Johnson Matthey skidded 3.1% to the bottom of the FTSE 100 after it said profit at its Clean Air business, which provides the lion's share of its earnings, would fall this year and said the unit's chief would step down.
Precious metals miner Fresnillo also slid 2.5% after it cut its annual production targets, citing lower-than-expected ore grades and construction delays at a gold mine in Mexico.
The falls more than offset gains for exporters like BAT and Diageo, which benefited from the pound's slide to a two-year low against the dollar, the product of both Brexit concerns around the Conservative leadership election and a run of U.S. data which has bolstered the greenback.
Seeking to woo voters on the right, both Boris Johnson and Jeremy Hunt have indicated they could take Britain out of the European Union without a deal at the end of October, potentially sparking chaos at British ports and hammering growth.
"It is increasingly looking like there is going to be something far scarier than ghouls and ghosts on Halloween 2019: a no-deal Brexit," Spreadex analyst Connor Campbell said.
Brexit jitters have taken a heavy toll on the housing and construction market, with mid-cap builders Galliford among those first in the firing line.
Its shares jumped 5.6% to the top of the mid-cap index on Wednesday, after it forecast annual pretax profit in line with analysts' estimates, a sign it was managing to ride out the worst of a downturn in housing sales and prices.
Luxury brand Burberry rose another 2.2% and was the biggest gainer on the main index after a slew of rating upgrades, a day after posting its biggest one-day gain on the back of robust first-quarter sales.
Among smaller stocks, Saga advanced almost 5% after activist investor Elliott took a stake in the specialist tourism and insurance firm, less than a month after it flagged concerns around its tours business.