In general BA will upgrade to Traveller Plus and Club ( generally not First ) to get staff away but they will not upgrade just because seats exist in that cabin although holiday spirits can often create more goodwill in respect of upgrades. Note that you will not be upgraded reqardless of the circumstance if you are not appropriately dressed i.e. Business casual at a minimum. Note that BA's dress standards are not as lax as those I've seen applied by some US carriers.