The British Airways B767 and most of the B747 fleet are approaching or are over 20 years old (and the Gatwick B737's are also getting on a bit) The orders are to replace the B767 and B747 (remember BA has wanted to replace the B767's for several years and would have done so by now had a provisional deal to convert them to tanker use not fallen through).
Not only are the new planes a lot more fuel efficient, the using the A380 on very busy routes (e.g New York) instead of 2 other planes frees up a slot which can be put to good use.