At the UK Phraseology Working Group (UKPWG) meeting where this was discussed, it was originally decided (using evidence from other countries) that there would be no requirement to add 'hectopascals' where the pressure was below 1000. Post meeting, one single ANSP reversed their opinion and objected, so now we're lumbered.
NB: Use of the word 'hectopascals' as opposed to 'millibars' is ICAO standard to which the UK filed a 'difference', however, EASA required the UK to comply as it's standard throughout Europe.