TonicQuake  I see two options for you.

OPTION A: Create two roles

This option works if you already have a security group that contains all the head office users and can add that in PowerBI.com

Basic User Role

This is what you already have - filters the data based on department.

Head Office Role

Create a second role that has no filters. Add the Head Office people to this role in PowerBI.com 

OPTION B: Create a security filter mapping table

This option works if you do not have a security group for the head office users, and keeps all the settings in Power BI desktop. Only one role is needed, but you'll need a more complex DAX filter in the RLS:

All Users Role

VAR _Role = MAXX(FILTER( UsersTable, UsersTable[UserEmail] = USERNAME() ), UsersTable[Department])
RETURN
IF( _Role = "Head Office", TRUE()
,
[UserEmail] =
USERNAME()
)