The blue-chip FTSE 100 fell 1.6%, after clocking gains of 12.4% in November, its best month since January 1989.
LLoyds Banking Group dropped 4.5% after appointing Charlie Nunn, currently head of wealth and personal banking at HSBC, as its next chief executive officer.
The domestically-focused FTSE 250, considered a barometer for Brexit sentiment, fell 0.7%, its best month since the global financial crisis in 2009.
"There is a feeling in the markets that pharma companies are closing in on the virus but there is still some way to go yet," said David Madden, market analyst at CMC Markets.
"It would appear that today's broadly bearish move in stocks has been driven by profit taking from recent gains and to a lesser extent, concerns about the health crisis itself."
Britain and the European Union are heading into a "very significant" week, British foreign minister Dominic Raab said on Sunday, as talks over the trade deal enter their final days with serious differences yet to be resolved.
"The expectation in markets is that there will be a deal," said James Smith, developed markets economist at ING in London.
The exporter-laden FTSE 100 has rebounded 30% from a coronavirus-driven crash in March, but until this month, had lagged its U.S. and European peers on fears about the extent of the economic damage from pandemic-related lockdowns.
Data showed confidence among British businesses fell to a four-month low in November due to new restrictions, but firms turned less pessimistic after news of a breakthrough in developing a vaccine.
Pets at Home Group Plc fell 1% after buying veterinary telehealth provider The Vet Connection for 15 million pounds ($20 million).
(Reporting by Shreyashi Sanyal, Shivani Kumaresan and Sagarika Jaisinghani in Bengaluru; Editing by Saumyadeb Chakrabarty and Tom Brown)
By Shreyashi Sanyal