AFAIK there is no hard rule stating BA must offer vacancies to internal bidders before external recruitment on to LH.
Priam rules state that if there are vacancies on a fleet where a pilot has a transfer bid into and that transfer is valid, i.e. not in initial type freeze or conversion freeze and able to bid, then that transfer is to be completed before external recruitment onto that fleet.
As the majority of the last recruitment was onto the SH fleets, with a 5 year initial freeze, which is finishing now for many SFO's then the internal bid this year will see quite alot of movement. Hence this recruiting will be for SH. Next year? Who knows it's anyones guess with the new aircraft coming along soon (ish).