The Office of National Statistics releases single-year life tables each year. These include an "average period expectation of life" (i.e., life expectancy). Note these single-year expectancies are different from the main life expectancy estimates, which are based on 3-year averages. You can read more about how these are calculated here and data for previous years here.
According to the ONS, "single year life tables give a more granular perspective on whether mortality patterns are improving, worsening or staying the same than three-year average life tables. However, unlike three-year life tables, single-year life tables are not National Statistics. They are considered less robust as they are more prone to annual fluctuations in deaths caused by seasonal events."
What will be the British male life expectancy at birth in 2021 according to the UK Office of National Statistics?
The prediction will be resolved using the 2021 version of "Single-year life tables - Great Britain edition of this dataset 2020" excel file published by the Office of National Statistics available here. There will be a 2021 tab in the next edition. Under the male heading, row age = 0 and column ex, British male life expectancy in 2021 should be reported.
The ONS released the data on the 22-24 of Sept for the past 4 years and will presumably stick with this pattern.