As already said above the RP in a workplace is "the employer" thus the departmental manger cannot be the RP.
I think Colin's definition of vicarious liability is a bit confused. Vicarious liability is normally (though not exclusively) a civil law principle, relying on the "master /servant" relationship and makes the employer responsible in civil law for the acts of his employee even though he may not be to blame for them. Individuals who break the law aren't vicariously liable - they are directly liable. In the Health and Safety at Work Act there are specific provisions (ss 7 and 37) allowing proceedings to be taken against individual employees and managers respectively.
It seems to me that Article 23 of the FSO (General Duties of Employees at work) was put in for the same purpose, i.e. to allow proceedings against an individual employee rather than the RP (the employer in this case). The wording of the Article almost exactly mirrors that of S7 of HASAWA:
1) Every employee must, while at work—
(a) take reasonable care for the safety of himself and of other relevant persons who may be affected by his acts or omissions at work;
(b) as regards any duty or requirement imposed on his employer by or under any provision of this Order, co-operate with him so far as is necessary to enable that duty or requirement to be performed or complied with
Article 32 (Offences) makes it clear that it is an offence to
fail to comply with article 23 (general duties of employees at work) where that failure places one or more relevant persons at risk of death or serious injury in case of fire;
This I suggest that in the hypothetical case the departmental manager could, if it were felt appropriate to do so, be prosecuted under Article 23. However he does not at any time become the RP. Neither in law does he become the employer just because he can hire and fire. In the case of a company, employer = the body corporate.