The benchmark index (.FTSE) declined 0.3%, hit by concerns around surging COVID-19 cases as the United Kingdom reported a record rise in daily infections.

British blue-chip shares gained 14.3% in 2021 but underperformed their European and U.S. peers, which have scaled multiple record highs.

"There are still concerns that the true impact of Brexit hasn't been very obvious just because of the COVID-19 pandemic and the logistics crisis caused by the lockdowns," said David Madden, market analyst at Equiti Capital.

"International fund managers are overall still taking money out of the UK and putting it elsewhere."

Mining (.FTNMX551020) and industrial stocks (.FTNMX502050) were among the top boosts to the FTSE 100 in 20201, with miner and trader Glencore (GLEN.L) and construction supplier Ferguson (FERG.L) both rising more than 50% in 2021.

Madden said heavy industries received a boost this year from a build of pent-up demand during lockdowns that far outpaced supply in this year's economic reopening.

"Industrials saw the orders jump to the roof and in turn they went into overdrive territory," he said.

Energy stocks (.FTNMX601010) posted their best year since 2016, ending a three-year slump, as the global economic recovery and producer restraint sparked a jump in crude oil prices.

The mid-cap FTSE 250 index (.FTMC) declined 0.3% on Friday due to a drop in consumer discretionary stocks such as Watches of Switzerland Group (WOSG.L), but was on track for its seventh consecutive quarter of gains.

UK markets closed early on Friday, the last day of 2021, and will remain closed on Monday.