Half the fleet are super hi J 747s, so half the destinations get them. But they do show up all over the place. As Ldnn1 noted, they rarely go from T3. Interestingly, I was told by a 747 captain that Lagos often gets the super hi J as it has a larger potable water tank than than the mid J, and they can't upload water in LOS, so need to fill it as much as possible in LHR. The loads rarely require the J seats though.