Shortly before midday, the FTSE 100 index was up 41.56 points, or 0.6%, at 7,041.52.
In Asian trade, the pound had sunk 6% against the dollar in a matter of minutes to $1.1841, before recovering.
It is not clear what triggered the flash crash, with some blaming automated trading systems reacting to a news report.
Sterling remained under pressure, and was still down 1.9%, more than 2 cents, against the dollar at $1.2378.
The pound has been sliding all week, with traders nervous about how the UK's talks with the EU over leaving the bloc will progress.
Last Sunday, Prime Minister Theresa May said she would trigger Article 50, the clause needed to start the exit process, by the end of March 2017.
A fall in the pound is often seen as beneficial to the FTSE 100 as many of the companies in the index generate most of their revenues abroad. A weaker pound means overseas revenues are worth more when they are converted back into sterling.
Mining companies were among the biggest gainers in the FTSE 100 on Friday. Shares in Randgold Resources rose 2.6% while BHP Billiton added 2.4%.
Shares in housebuilders were coming under pressure, however, with Barratt Developments down 5% and Taylor Wimpey 4% lower.
In the FTSE 250, shares in William Hill and Ladbrokes were both lower after a report in the Times said gambling companies could be banned from advertising on TV during the day.

Ryan Counsell, 28, a Muslim convert from Nottingham, denies four charges, including having possession of an al-Qaeda bomb-making guide.
He told Woolwich Crown Court on Tuesday he downloaded the videos due to a "business interest".
He said he wanted to create a "catalogue of propaganda" online.
More on this story and other news in Nottinghamshire
The married father, who worked at the Hyson Green branch of Asda in Nottingham, told the judge he downloaded videos so he could analyse and write articles about them.
The jury was shown several of the videos, including footage of explosions and material explaining the structure of the caliphate (an Islamic state).
When asked to explain why he watched this video he said it was something he found "interesting".
He said its "main purpose" was "to show how Islamic State claim to organise themselves".
Asked why he had named a folder of material on his computer entitled "alleged Jihad", he told the jury: "It is something that I do not believe they are actually performing with a genuinely religious motivation.
"I believe it's just a form of terrorism, it is nothing to do with the religion. That's why I titled it alleged Jihad."
Counsell is charged with preparing acts of terrorism by obtaining information about Islamic terrorist groups in the Philippines, arranging to travel to the Philippines to join and fight for an Islamic terrorist group, and purchasing equipment and clothing for use when having joined such a group.
The trial continues.
London's main share index headed higher as the pound continued to slide on the foreign exchanges.
[NEXT_CONCEPT]
A supermarket worker accused of trying to join a terrorist group watched Islamic State (IS) videos to learn more about propaganda, a court has heard.