The standard praseology given in MATS 1 for what seems to be going on is:
'When established on the localiser, descend on the ILS, QFE (Pressure) Millibars/QNH (Pressure) Millibars, Elevation (Number) feet.'
Presumeably preceded by 'Closing the localiser from the left/right' although it doesn't say it quite as clearly as it might.
Seems straightforward to me.
If you wanted a theoretical answer, I think I'd say that the aircraft should maintain the last cleared level until specifically cleared for the approach (which in today's phraseology is indicated by 'descend on the ILS ').