Yep, MATS part1 states that the word 'millibars' may be omitted when the pressure is >=1000mb. It doesn't say that it is incorrect to use the word millibars at all times.
When I was a green trainee I decided to use the word 'millibars' at all times. I had to stop this practice, as in the eyes of the instructors of the day, by using the word 'millibars' at all times, one couldn't prove to them that you appreciated that in certain circumstances it was required and in other circumstances it was not.(I guess that they thought we had just fluked the written exams) Trainees are 'chopped' on such issues. Crazy I know, but true!
Shortage of ATCOs? Now you know why.