BA Holidays almost always booked in I, C or D unless you explicitly select 'fully-flexible' in the search. Of course, since they also offer 90% refund on a cancellation of the whole package (ie deposit of 10%), there's less reason to pick flex. So I think thread title is wrong, it's not J fares, those would be much higher than the 2 x 120 quid extra for seat choice.

Incidentally, if you are flying from outside UK, e.g. Dublin or such, BAH does the connecting flight in J and you can select seats for it.

BAH could be clearer about it, but before you pay your deposit, the overview of flights mentions if you do have seat selection, but not if you don't...