.Ryanair has it right. Seniority based on the day you join. Seniority as a captain based on the date you pass your command upgrade. It would solve the problem we have regarding SH command
It depends what exactly you think the "problem" is but I suspect such a scheme might have consequences for those who join or who have joined BA hoping for a fast SH command
In passing just an observation that I think might be worth making to avoid any tendency to stereotype parts of the BA workforce:
Over the years at BA there have been more than a handful of "long term" Long Haul Co-pilots who have successfully gone to Short Haul for a command by way of a Command conversion.