* FTSE 100 about flat
* FTSE 250 up 0.2 pct
* Oil majors lead blue-chips higher
* Standard Life Aberdeen rises after results
* BAT, Imperial Brands cap gains on main index
* GVC slips to >2-1/2 yr low
* Avast drops after earnings miss, CEO leaves (Adds company news items, updates to closing prices)
By Shashwat Awasthi
March 13 (Reuters) - Britain’s main stock index ended roughly flat on Wednesday with gains in oil majors offset by a drop in tobacco stocks and as investors remained cautious before of a second pivotal Brexit vote of the week.
The FTSE 100 was flat while the FTSE 250 was up 0.2 percent.
Oil heavyweights Shell and BP gained on the main bourse as oil prices rose on ongoing supply cuts from producer group OPEC and U.S. sanctions against Iran and Venezuela.
The index was also supported by Standard Life Aberdeen which rose 2.4 percent on its best day since early January after posting full-year results and saying that it was doing away with a dual-CEO management structure.
Lawmakers voted against Prime Minister Theresa May’s amended Brexit deal by 391 to 242 on Tuesday, forcing parliament to decide whether to back a no-deal Brexit or seek a last-minute delay to the process.
“This is all very messy - there is no clear and clean way out of this - leaving uncertainty and caution the order of the day for sterling and other UK assets,” Markets.com analyst Neil Wilson said.
Parliament will vote later on Wednesday on whether to leave the bloc with no deal. To that end, May has said she will vote against a no-deal exit.
If a no-deal exit plan is rejected, a vote on Thursday will decide whether to seek to extend the Brexit deadline.
“While this is something that might well be easy to achieve as it kicks the can down the road, it is by no means certain that the EU might feel obliged to accommodate it, if the reason behind the request doesn’t have an endpoint goal behind it,” said CMC Markets analyst Michael Hewson.
Gains on the main index were limited by tobacco firms British American Tobacco and Imperial Brands, which fell after U.S. National Cancer Institute Director Norman Sharpless was named as acting U.S. Food and Drug Administration chief.
Gambling firm GVC skidded 4.6 percent to its lowest level since June 2016.
Drugmaker Hikma shed 4.3 percent after its full-year core operating profit missed estimates.
On the mid-caps, G4S, the world’s biggest security firm, lost 4.6 percent as RBC cut price target after what the brokerage called “messy” results and said it was sceptical of the company’s plan to separate its cash business.
Cyber security firm Avast dropped 3.3 percent after its long-time CEO stepped down and it reported marginally lower-than-expected adjusted revenue and earnings in its first annual results since listing last May.
But oilfield services provider Petrofac jumped as much as 5.5 percent after it was awarded a $1 billion project in Algeria. (Reporting by Shashwat Awasthi in Bengaluru; Editing by Keith Weir and Andrew Heavens)