The United Kingdom celebrates its armed forces on the last Saturday of June — which is 24th this year. A week before the holiday, flags of the armed forces are raised across the country, and on June 21st a Reserve Day is held: the UK celebrates its veterans, cadets, and civil servants alike!
From June 23rd (11:00 GMT) until June 26th (7:00 GMT), play 3 battles using British vehicles (rank III or higher, with an activity of not less than 70%) to receive the “British Armed Forces Day” decal.
Complete the task in random battles, except for Enduring Confrontation missions and [Assault] mode. Track your progress in the hangar by clicking Nickname → Achievements → Holidays → British Armed Forces Day.
You can find the decal in the “Holidays” tab in the Customisation menu.