It is the Emergency Oxygen Manual Operating Handle

The emergency oxygen system has a gaseous oxygen storage bottle fitted
to the back of the ejections seat which feeds its supply through an
emergency oxygen demand regulator to the rear of the seat portion of
the PEC [Personal Equipment Connector] and thence into the oxygen mask tube. Emergency oxygen is
selected manually by pulling the emergency oxygen knob, or
automatically on ejection as the seat rides up the rails. When the
pilot separates from the seat after ejection, the separation of the
man and seat portions of the PEC breaks the emergency oxygen supply
and allows air to be breathed at ambient pressure.
source - English Electric Lightning F Mk 36 & T Mk 5 Flight Manual