The BA 21/22 was a complete disaster. The loads were poor and the revenue was even worse as the premium cabins were often half empty (especially on the HKG to LHR sectors). If you look back you will find that often the 21/22 rotation was cancelled and indeed it was cancelled for the last month of the 6 months that it was supposed to operate for this reason.

If BA wants more capacity to HKG then given the 25/26 has the biggest bird in the fleet then the 27/28 will follow suit first before BA even consider a 3rd rotation. 2xA380's might seem bizarre but suspect that this is more likely than a 3rd flight....

FD.