The FTSE 100 slipped 0.6 percent by 0840 GMT, well off a six-month high it held in the last two sessions, while the FTSE 250 was 0.4 percent lower.
Losses on the main index were across sectors, with CMC Markets' analyst David Madden saying it seemed like investors were booking profits as the markets have seen a strong week so far on easing fears over a no-deal Brexit and growing hopes of a resolution to the Sino-U.S. trade dispute.
"It's not really a surprise that the markets has pulled back ever so slightly today."
On the Brexit front, a law was narrowly passed in the lower house of parliament to seek a second delay to Britain's divorce from the European Union, igniting optimism that a disorderly no-deal exit could be prevented.
Sterling inched up following the vote, along with housebuilders, considered among the most exposed sectors to a hit to the economy.
But that was not enough to offset steep losses in financials Lloyds, Direct Line and St James's Place , which traded ex-dividend.
BP and Shell slipped after crude prices weakened after a surprise buildup in U.S. inventories last week.
Marring the mood further was data coming out of Germany, Europe's largest economy, which showed industrial orders fell at their steepest rate in more than two years in February, hit by a slump in foreign demand.
Saga, the over-50s tourism and insurance specialist, contributed a fourth of the losses in the FTSE 250 after it warned on profit and slashed its dividend amid a highly competitive motor and home insurance market.
"(This is a) kitchen sink job, it looks nasty," one trader said of Saga, whose shares were last down 38 percent at its lowest ever.
A bright spot was Britain's largest industrial distribution firm Electrocomponents which added nearly 3 percent after reporting a jump in like-for-like revenue in the fourth quarter and saying full-year adjusted earnings will be in line.
Entertainment One, maker of children's TV show Peppa Pig, and home repairs provider HomeServe both edged higher by 2 percent after strong earnings updates.
But online electricals retailer AO World slumped 9 percent to the bottom of FTSE small-cap index after saying it expects adjusted core earnings for the year to be at the lower end of market view.
Baby products seller Mothercare also shed 6 percent after flagging challenging market conditions and reporting a fall in UK like-for-like sales for the last quarter of its fiscal year.