The 13 757's that BA operate on SH are expected, in todays plan, to stay until 2008 at least. This of course means that the 321's are in addition to this, hence the need for plenty of new blood. Some of the 319's at LHR are planned to move to LGW at some point so it could be the 737 which sees the reduction.
Retirements do slow down drastically in the next couple of years and if the retirement age goes up to 60 in 2006 things could stagnate again. This opportunity could be relatively short term, when the advert comes out and you fancy a go, don't hang around! Good luck