DEP LH has been offered in the past - as you say, '96-'97 certainly. In fact I was offered 747 but pushed for 757/767 and got it.
LGW Commands were effectively "DEC" about then as well... as you say, it would be via a LHR fleet, but the LGW T&Cs poor enough, and rate of progression/retirements elsewhere, many took it soon after joining.
Since then the 65 retirement age, stagnation, improved LGW T&Cs mean the LGW Command time is now high (but not as high as LHR of course), and we have plenty of SH P2s desperate to get to LH and eligible. That is why LH DEP recruitment might cause a few ripples
