Greetings Loc green,
you were right on spot concerning the "flex" meaning.
SRS means speed reference system which is responsible for maintaining min. V2+10 (2-eng) or current speed or V2 (whichever is higher) OEI. Included is also an attitude protection, windshear protection and flight path angle protection. It disengages automatically at acc.alt or at FCU-selected altitude or manually.
RWY guides the aircraft on rwy centerline for the t/o-roll and initial climb to 30ft if a LOC signal is available on the runway.
Hope this helps,
MachineMan