* FTSE 100 stretches winning streak to seven sessions
* FTSE 250 up 0.3 pct
* Oil, miners boost main index
* Ocado shrugs off Andover fire to hit life high
* Antofagasta rises on higher-than-expected dividend payout (Adds news items, updates to closing prices)By Shashwat Awasthi and Yadarisa ShabongMarch 19 (Reuters) - Britain's FTSE 100 extended its winningstreak to seven sessions on Tuesday as miners and oil majorsboosted the index and Ocado climbed to an all-time high afterposting higher first-quarter retail sales.The FTSE 100 was up 0.3 percent. The index held onto its five-month high and is now on its longest streak of gainssince May 2017.The domestically-focused FTSE 250 was up 0.3 percentas investors waited for further Brexit developments with reportsthat Prime Minister Theresa May would ask the European Union todelay Brexit by at least three months. May's Brexit plans were derailed after the speaker ofBritain's parliament said on Monday that her deal could not bevoted on again after two thumping defeats unless a differentproposal was submitted.Sterling, which fell after Tuesday's Brexit update but laterrecovered losses, remained steady as investors awaited furtherclarity on the state of Britain's divorce from the EU."You must say the sanguine reaction in currency marketsreflects the fact that no one really knows where this leaves theBrexit story," Markets.com analyst Neil Wilson said.Heavyweight oil majors helped lift the main index as supplycuts by OPEC and U.S. sanctions against Iran and Venezuelabuttressed prices, while miners rose on China'splan to cut tax for manufacturers from April.Online supermarket Ocado , added 5.4 percent to hitan all-time peak after posting a double-digit jump infirst-quarter retail sales, despite growth being hit by lastmonth's fire at a large distribution centre. Chilean copper miner Antofagasta advanced 2.8percent, touching a seven-month high, as ahigher-than-anticipated dividend payout overshadowed a drop inannual core earnings. Drugmaker Hikma was 2.5 percent higher after arating upgrade from Citi and positive comments on growthprospects from Jefferies, while events manager and publisherInforma rose 3.7 percent on its best day in nearly 18months after a Morgan Stanley upgrade.Sainsbury's ended 0.9 percent higher. Britain's No.2 supermarket chain and Walmart's Asda committed todeliver 1 billion pounds of annual price cuts in an attempt tosalvage their proposed merger after being dealt a potentiallyfatal blow by the country's competition watchdog. Mid-cap oilfield and engineering services provider WoodGroup tumbled 8.1 percent after warning that itsdeleveraging plan would be more gradual than originallyanticipated. But, TP ICAP , the world's largest interdealerbroker, climbed 4.9 percent higher on its best day since August2017 after its full-year underlying pretax profit beat analysts'estimates. (Reporting by Shashwat Awasthi and Yadarisa Shabong inBengaluru; Editing by Keith Weir and Mark Heinrich)