The mid cap index added 0.4%, while the FTSE 100 fell 0.3%, with AstraZeneca marking its worst day in nearly six months with a 3.8% fall.
AstraZeneca said the average efficacy of its vaccine, which is being developed with the University of Oxford, was at 70% in its late-stage trial.
Although the British drugmaker said its potential vaccine can be 90% effective, its average rate was lower than for Pfizer-BioNTech's and Moderna's, which showed efficacy rates of about 94% to 95%.
"Even though the late stage trials were better than expected, AstraZeneca's shares didn't get a booster shot," Susannah Streeter, senior investment and markets analyst, Hargreaves Lansdown, said of the news.
"It has though added to the overall positivity washing around the financial markets about a faster end to the pandemic."
Britain's health minister said the country aims to roll out a COVID-19 vaccine before Christmas and ease restrictions next month, raising hopes of a sooner-than expected recovery and boosting consumer stocks.
While positive vaccine-related headlines have helped British stocks gain this month, they have still underperformed global peers due to uncertainty over Brexit negotiations, as the end of year deadline for a trade deal looms.
(Graphic: Brexit woes keep UK stocks global laggards - https://fingfx.thomsonreuters.com/gfx/mkt/qmyvmxxqzpr/brexit%20and%20uk%20stocks.PNG)
While data showed that British business activity contracted in November due to a new wave of coronavirus lockdowns, news of possible vaccines sharply boosted hopes for 2021.
In corporate news, Cineworld Group jumped nearly 20% after it secured waivers for its debt covenants until June 2022, along with $450 million in new loans.
(Reporting by Devik Jain in Bengaluru; Editing by Saumyadeb Chakrabarty and Alexander Smith)
By Devik Jain and Ambar Warrick