The IRIS site says:

If you are in Heathrow Terminal 3:

The enrolment room is before you reach the Duty Free shopping area, opposite the BAA Security desk.

This enrolment room will normally be open between 0800 to 1830 and 1900 to 2030.

Unfortunately there may be on occasion the need to close enrolment rooms at short notice for operational reasons. Such closures will be kept to a minimum and will be as short as possible.

Is this Arrivals or Departures? Landside or Airside?

Thanks.