From the FAA/Pilot/Controller Glossary:
"RUNWAY HEADING− The magnetic direction that
corresponds with the runway centerline extended, not
the painted runway number. When cleared to “fly or
maintain runway heading,” pilots are expected to fly
or maintain the heading that corresponds with the
extended centerline of the departure runway. Drift
correction shall not be applied; e.g., Runway 4, actual
magnetic heading of the runway centerline 044, fly
044."
From CAP 413:
"Straight Ahead When used in departure clearances means: ‘track extended
runway centre-line’."
There appears to be no "Runway Heading" phrase in CAP 413 so it should not be issued as a clearance within the UK.
RP20