Originally Posted by UK CAA, CAP 413 2.69
ATS messages ... to be read back in full by the pilot ... If a readback is not received the pilot ... will be asked to do so.
-Altimeter Settings, including units when value is below 1000 hectopascals
dated MAY 2016, extracted from the website.
Originally Posted by ICAO Doc 4444 PANS-ATM 4.5.7.5 READBACK OF CLEARANCES
4.5.7.5.1 The following items shall always be read back: ...
c) ... altimeter settings ...
4.5.7.5.2 The controller shall listen to the readback to ascertain that the clearance or instruction has been correctly acknowledged by the flight crew and shall take immediate action to correct any discrepancies revealed by the readback.
Fifteenth Edition — 2007 AMDT 5
The mandatory read-back of units below 1000 mb seems to be a UK thing - I did rather thorough search of the ICAO paperwork.
I would consider it a constitucionalized best-practice against confusion with american operators used to inches, for reasons discussed and explained above. Anyone suffering from this UK requirement is actively participating on a safety net preventing trans-atlantic aircraft crashing into other traffic. Let's now make an informed decision how much to get aggravated
The rule was first implemented in CAP 413 edition 15 amendment 01 in December 2004, together with the "degrees" added after heading values ending in zero. Similar requirement exists for ATC personnel communicating over telephone, intercom of even relying company messages - readback and its verification included.
Today's wording (since JUL 2013 or older)
Originally Posted by CAP 413 Chapter 3 General Phraseology
3.4 For all transmissions, the word ‘hectopascal’ shall be appended to figures when transmitting a pressure setting below 1000 hPa, or in cases where confusion or ambiguity may result.