It's what's called a NOTAM or 'Notice To Airmen'. I think you'd need to register to see the site. Here is the text.
RESTRICTED AREA (TEMPORARY). RESTRICTION OF FLYING REGULATIONS
MADE UNDER ARTICLE 161 OF THE AIR NAVIGATION ORDER 2009. NO ACFT IS
TO FLY WI THE AREA BOUNDED BY STRAIGHT LINES JOINING THE FOLLOWING
POINTS 514508N 0001309E - 514055N 0000652E - 513232N 0000055W THEN
THE ANTI-CLOCKWISE ARC OF A CIRCLE HAVING A RADIUS OF 17 NAUTICAL
MILES CENTRED ON 512812N 0002713W FROM 513232N 0000055W TO 513611N
0000311W THEN STRAIGHT LINES JOINING THE FOLLOWING POINTS 513611N
0000311W - 513611N 0001830W - 515101N 0000025W - 515146N 0000006W -
515155N 0000120E THEN THE ANTI-CLOCKWISE ARC OF A CIRCLE HAVING A
RADIUS OF RADIUS 8 NAUTICAL MILES CENTRED ON 515306N 0001406E FROM
515155N 0000120E TO 514508N 0001309E. THESE RESTRICTIONS DO NOT APPLY
TO ANY AIRCRAFT MAKING AN APPROACH TO, OR DEPARTING FROM, LONDON
STANSTED AIRPORT WHILST UNDER THE CONTROL OF THE LONDON TERMINAL
CONTROL CENTRE AT SWANWICK OR OPERATED BY THE NATIONAL POLICE AIR
SERVICE OR THE HELICOPTER EMERGENCY MEDICAL SERVICES OR OPERATED WITH
THE AUTHORITY OF CHIEF INSPECTOR VINCE BRADY OF THE METROPOLITAN
POLICE SERVICE WHO MAY BE CONTACTED THROUGH THE NATIONAL POLICE AIR
SERVICE AT LIPPITTS HILL ON TELEPHONE 020 8345 4888. FULL DETAILS
INCLUDING A CHART CAN BE FOUND AT
WWW.NATS-UK.EAD-IT.COM/PUBLIC/INDEX.PHP.HTML. AUS 16-04-0166/AS6