My understanding is that there were a small number of pilots who received such pay rises. These individuals were senior Concorde pilots who around that time, moved to longhaul due to the aircraft going out of service. People who flew Concorde, did not do so for the money, hence it was not especially well paid. Transferring to a better paid fleet was by far the biggest contributor to those individuals' 20% pay rise.
Quoting 20% pay rises for the pilots, is basically like the cabin crew getting a 3% pay rise across the company one year, and then quoting that you got a 20% pay rise because a few crew transferred from LGW to LHR long haul around the same time, with the associated increase in their pay.
It's a good figure to throw around, to try to create envy, and to try and get yourself a payrise or defend your pay, but in reality, these pilots achieved their large pay rises by effectively changing jobs.