Odd. I took BA26 last year and boarding for the upper deck was through U1L. I was in row 53 and didn't see anyone use the stairs.
^
I have been using the A380 once a month LHR-HKG-LHR since it started and it has never been down the stairs except once when the upper jet bridge failed to operate. I also have been told the stairs are not used by passengers as it would be too noisy for the F passengers below.
If it is the first time in CW on the 380, go upstairs as the bathrooms are massive.