Blue chip shares were on the front foot on the historic day that Britain triggered Article 50, signalling the end of the UK's 44-year relationship with the European Union.
The FTSE 100 Index lifted 21.4 points to 7,364.8, shrugging off at least two years of uncertainty as Britain begins talks on new trading relationships with the soon-to-be 27-member European bloc. The FTSE 250 edged up 4.5 points to 18,957.8.
CMC Markets Chief Market Analyst Michael Hewson said: "As a piece of symbolism the picture of the UK's EU ambassador Tim Barrow handing over the official notification of the UK's intention to leave the EU over to EU Council President Donald Tusk is a powerful one, but in terms of the effects on financial markets today its impact has been minimal, with the pound coming under some selling pressure, but no more or less than in previous sessions."
In London, the market was dominated by Brussels confirming it will block the €29bn (£24.6bn, $31bn) merger of the London Stock Exchange (LSE) and Deutsche Börse on competition grounds hours before the UK served its European divorce papers.
EU competition commissioner Margrethe Vestager scotched the third attempt in 17 years to merge the Anglo and German stock exchanges after they refused to meet her requests to sell off the LSE's Italian bond trading platform called MTS.
Vestager said: "The merger would have led to a de facto monopoly."
However, shares in LSE were among the highest risers in the top-flight lifting 3%, or 89p to 3,113p, as traders bet this makes the UK exchange vulnerable to takeover bid from a US rival.
The biggest afternoon risers in the FTSE 100 Index were 3i Group (+32.5p to 733p), London Stock Exchange (+89p to 3,113p), BHP Billiton (+28.5p to 1,255.5p), Ashtead Group (+28p to 1,632p) and Associated British Foods (+41p to 2,653p).
The biggest afternoon fallers in the FTSE 100 Index were Fresnillo (-28p to 1,534p), Barratt Developments (-8p to 538p), Randgold Resources (-105p to 7,065p), Standard Life (-5p to 355.3p) and TUI (-15p to 1120p).
The biggest afternoon risers in the FTSE 250 Index were Nostrum Oil & Gas (+29.1p to 461p), Evraz (+8.8p to 226.8p), Stagecoach (+7.3p to 206.6p), Sanne Group (+18p to 648p), Entertainment One (+5.7p to 230.1p).
The biggest afternoon fallers in the FTSE 250 Index were Polymetal International (-58.5p to 981.5p), Berendsen (-33.5p to 727.5p), AA (-10.6p to 260p), JRP Group (-4.4p to 134.5p) and Sophos Group (-7.8p to 272.2p).