Both the FTSE 100 and the FTSE 250 ended 0.1 percent lower, although the blue-chips trimmed losses after data showed U.S. economic growth accelerated in the first quarter.
Mining company Glencore fell 3.3 percent after saying that the U.S. Commodity Futures Trading Commission was investigating possible "corrupt practices" that may have broke some rules.
Royal Bank of Scotland skidded 4 percent, its biggest fall in nearly five months, as it reported lower first-quarter profit that was hurt by intensifying competition and Brexit uncertainty.
Oil majors Shell and BP weighed heavily on the bourse as U.S. rival Exxon Mobil slumped on the Wall Street after reporting a 49 percent plunge in quarterly profit due to weakness across its major businesses.
A fall in crude prices, with the market retreating from its strongest bull run in at least a year, also did not help oil shares.
Online takeaway service Just Eat fell 4.6 percent after reporting slower growth in UK orders during the first quarter, which it blamed on warm weather in February and a later Easter this year.
"Weather has always been a factor for Just Eat, and we remain convinced that there are solutions to that, but they haven't yet been put into place," Peel Hunt analysts said.
Helping contain losses on the main index was the world's largest advertising company, WPP, which enjoyed its biggest one-day rise in nearly a year with a 5.4 percent jump after reporting higher revenue and reiterating its annual targets.
While the FTSE 100 had dropped for the last three sessions, it still remains on course for its fourth straight month of gains.
The mid-cap index is on track to post monthly gains as well, despite the re-emergence of Brexit uncertainties.
Goldman Sachs said Brexit's effect on the British economy was intensifying as the process of leaving the European Union drags on.
Ferrexpo plummeted 28.4 percent on its worst day in 3-1/2 years after Deloitte abruptly quit as its auditor.
The stock drop came as the iron ore pellet producer said some funds of a charity it donates in Ukraine might have been "misappropriated".
Insurer Hastings slumped 12.8 percent, its steepest drop in six months, after it forecast an annual loss ratio - the amount it spends on claims compared with income from premiums - towards the higher end of its target range.
Its blue-chip peers Admiral Group and Direct Line gave up 1 percent and 1.2 percent, respectively.
Computacenter surged 18.3 percent on its best day in nearly a decade as the IT services company stuck to its annual targets after a "pleasing" first quarter.
Glencore underperforms mining heavyweights:
(Reporting by Shashwat Awasthi and Muvija M in Bengaluru; Editing by Janet Lawrence, Larry King and Frances Kerry)
By Shashwat Awasthi and Muvija M