There are two toilets on the upper deck of a BA
747-400, one at the front of the cabin (next to the flight deck door), one at the rear (adjacent to the staircase), and both are for the use of passengers.
60J is one of the most popular seats on the upper deck and is rarely empty.
60A and 60B are not always available on ultra-long range flights, as they are blocked off for crew rest
As has been said, row 62 is very popular, and seats 62A and 62K tend to be requested early by those passengers who are travelling alone.
For a couple, if you can't get 62A/B or 62J/K, I would go for 60J/K.
Hope you enjoyed the flight wherever you sat!