The FTSE 100 edged 0.2pc lower, with losses spread across most sectors, and the FTSE 250 midcap index was down 0.4pc.
The oil and gas sector was a bright spot, with heavyweights Shell and BP gaining on the back of a jump in crude prices after a report that US President Donald Trump had approved military strikes against Iran then pulled them back.
"Overtones of the trade conflict are, like the unseasonably wet weather in much of Europe right now, difficult to avoid and likely to put a dampener on things," City Index analyst Ken Odeluga said.
"The simmering and increasingly unpredictable US-Iran stand-off has also chilled the week's revived cheer."
After a solid run during a week in which the US Federal Reserve and the European Central Bank signalled more stimulus if the economy weakens, there seemed to be some profit taking in markets, CMC Markets analyst David Madden said.
The gains from the earlier in the week have placed the FTSE 100 on course for its best month since January, after it suffered its first monthly fall in May this year due to escalations in the US-China trade war.
Keeping in mind the upcoming meeting between Presidents Donald Trump and Xi Jinping next week at the G-20 summit, Madden said there could be some more profit-taking ahead.
IQE Plc, which makes semiconductor wafers for chips, slumped 24.8pc on its worst day since November after warning annual revenue would be lower than expected because US restrictions on China's Huawei are affecting orders.
The alert dragged down European peers, and came after US chipmaker Broadcom rattled the industry with a forecast that US-China trade tensions and the curbs on Huawei would wipe $2 billion off its sales this year.
Another semiconductor firm, Nanoco, plummeted 74pc, its biggest one-day drop on record, after it said a US customer had informed it that its project with the company will not continue beyond their current contract.
Among midcaps, Domino's Pizza Group fell 3.7pc after a frontrunner to replace its outgoing chief executive slapped down a media report that he would step into the role.
Andrew Rennie insisted he was invested in his job as European head of Domino's Pizza Enterprises.