As I understand it the chronology of events will follow in roughly this way,
1. BA will firm up their route structure for winter '12/'13 and summer '13. Once this is complete they will know which aircraft will be flying which routes and therefore how many pilots they will need to man/woman those aircraft.
2. The first 787s and A380s arrive in the spring of next year so pilot on the present fleet will be moving across. There will also be vacancies created by promotions, sicknesses, pregnancies, people going part time and possibly the start soon of retirements.
3. Present pilots then have the opportunity to bid for those vacancies in the autumn. When they move they will then leave an empty slot behind them.
4. Only at this stage will BA know how many vacanies they have for external candidates and in what time scale. Obviously any vacancies arising prior to the FPPs finishing will need to be filled by DEPs and as has already been said, any med/long haul slots after this time will also require DEPs.
Hope this is some use.
Regards