The BA seniority scheme protects you if you start SH and then move to LH. If you spend 5 years on the bus and then apply for LH and your bid is valid then you will transfer. If there has been an influx of DEP pilots a year before you will still be above them in bidline and therefore have the ability to pick your work.
Seniority is king in BA so hanging back for LH from the start could cost you dear.