Twenty 4 Seven
There have been no promotion opportunities for crew who joined BA in 1997 on WW. As SlideBustle says - they have been too junior to apply. Either your purser was:
1. From LHR EF - fleet for European destinations.
2. From LGW WW - they closed in 2005 and most crew transferred to LHR WW.
3. A main crew who had been pulled out from standby to "work up" as a purser.
My bet would be either 2 or 3 - because there haven't been that many transfers for pursers between EF and WW and vice versa - and when there has been many of them have been very senior.