Britain's top stock index sank on Wednesday as mining companies and oil majors sold off amid growing anxiety that global growth was slowing.

The FTSE 100 fell 0.5 percent by 1000 GMT, hit by a commodities selloff and nerves over Brexit.

Sterling surged late on Tuesday after Britain and the European Union agreed a draft Brexit proposal, but it weakened on Wednesday as traders prepared for UK Prime Minister Theresa May's showdown with her cabinet.

"Markets are struggling to discount two binary options, a Brexit which is relatively soft and pushed down the road or a dramatic no-deal result," said Edward Park, investment director at Brooks Macdonald.

"Whilst a deal remains the market’s base case, the threat of a no-deal sword of Damocles hangs over UK assets until the start of 2019 at the earliest."

Sterling's overnight volatility jumped to its highest since the 2017 UK general election, Refinitiv data showed.

Figures showing inflation failed to rise as expected in October sent sterling slightly lower, alleviating some of the pressure on the FTSE 100.

Mining companies and oil majors dragged the FTSE down the most. Commodities-related sectors were hit by worries over slowing global growth and excess supply of crude.

Copper miner Antofagasta fell the most, losing 4.7 percent, as copper prices slid on weak China retail sales data.

Rio Tinto, BHP Billiton, Evraz, Anglo American, and Glencore fell 2.5 to 3.6 percent.

Oil majors BP and Shell also tumbled after crude prices plunged 7 percent on Tuesday. The International Energy Agency said global oil supply will outpace demand throughout 2019.

Smiths Group was among a handful of gainers, up 4.6 percent. The British engineer said it planned to spin off its healthcare business to focus on industrial technology, two months after a 7 billion-pound ($9.1 billion) deal to merge it with U.S.-based ICU Medical collapsed.

Micro Focus  shares rose 5.7 percent after Goldman Sachs upgraded it to "buy". The company also said it had bought back 213,400 of its shares as part of a $400 million buyback programme.

Mid-caps fell 0.3 percent.

Debenham shares tumbled 12 percent with analysts at Berenberg pointing to a report some high street suppliers have stopped working with the department store chain. 

Overall, analysts have been downgrading their expectations for UK earnings this results season, and investors say company reports show evidence of growing margin pressure and strain from preparations for Brexit.

"The UK earnings season has again shown the precarious state of the UK retail market which has struggled to pass on cost pressures to consumers," said Brooks Macdonald's Park.

"Exporters who are wrestling with the practicalities of European distribution in the event of a no deal have delayed investment and this has contributed to suppressed earnings expectations for 2019," he added.