Investors failed to take heart from a £1.3 billion takeover deal and the bullish performance of Asian markets as shares slipped back.
Traders seemed to remain focused on the prospect of military intervention in Syria as the FTSE 100 Index dipped 16.6 points to 6530.7.
GlaxoSmithKline's sale of soft drinks brands Ribena and Lucozade to Japan's Suntory pointed to more optimism around big mergers and acquisitions, after Vodafone's recent £84 billion exit from America.
Yet the UK-based pharmaceuticals multinational saw shares fall 11p to 1640p amid the gloom about the wider world.
Earlier, the Nikkei 225 jumped 2.5% after Tokyo was chosen to host the 2020 summer Olympics and it emerged that Japan's economy grew faster than previously thought in the second quarter.
The world's third-largest economy grew by 3.8% year-on-year between April and June, up sharply from an initial estimate of 2.6%.
Meanwhile, Chinese data showed that the country's exports accelerated and inflation edged lower last month, raising hopes for a pick-up in the world's second-largest economy.
But Michael Hewson, senior market analyst at CMC Markets UK, said investors were still digesting remarks from Syria's president Bashar al-Assad that he would strike back if attacked.
European bourses were also subdued, with Germany's Dax flat and France's Cac 40 falling slightly. However, New York's Industrial Average shook off anxieties as it climbed in early trading as the market took the positives from Chinese data.
On the currency markets, the pound rose against the greenback as the likelihood of a tapering of stimulus from the US Federal Reserve eased. It follows last week's disappointing jobs figures.
Sterling was up one cent at 1.57 US dollars and flat against the single currency at 1.19 euros.
Meanwhile in London, another strong sales performance at budget fashion retailer Primark failed to lift shares in owner Associated British Foods, which fell nearly 2% or 33p to 1818p following a rise of 16% so far this year.
Traders were spooked by ABF's warning over tough talks with European customers on sugar prices, where it is under pressure from a sugar glut and low global prices.
Food ingredients firm Tate & Lyle joined it on the fallers board, after UBS downgraded its price recommendation on the blue-chip stock on risks to sucralose sweetener prices, leaving it more than 3% or 27.5p lower at 779.5p.
Oil and gas exploration group BG was the biggest faller in the FTSE 100 Index after it lowered its production guidance for the next financial year, partly due to ongoing instability in Egypt. Shares were down 5% or 65p to 1217p.
Luxury fashion brand Burberry advanced 2% or 35p to 1635p after an upgrade from HSBC, which is more optimistic on its growth prospects.
The biggest risers on the FTSE 100 were ARM Holdings, up 25p to 917.5p, TUI Travel, up 9.2p to 347.4p, Aviva up 8.8p to 399.8p and Burberry up 35p to 1635p.
The biggest fallers on the FTSE 100 were BG Group, down 65p to 1217p, Tate & Lyle, down 27.5p to 779.5p, Associated British Foods down 33p to 1818p, and Randgold Resources off 75p at 4914p.