Since 1875 the UK market has seen negative annual returns only twice in the fifth year of the decade. As can be seen in the accompanying chart, since 1875 the down years were 1915 and 1945 (and 1945 was down only 0.6%).
As of the close 24th December the FTSE All-Share index was at 3449.5, (2.4% below its 2014 close of 3532.7).
The market has three days to climb above 3532.7 and end the year in positive territory.
The answer to the heading is: yes. The FTSE All-Share index ended 2015 at 3447.46: down 2.41% on the year.