Oh dear, you really are very new to BA Slow Descent, and I suspect new to this flying business too. Let me explain some things to you. The CAAs CAP371 limits are not based on some research into fatigue, they are based on years of research into fatigue, research which is still ongoing and is some if the most thorough in the field. When the bidline rules limits were thrashed out in the good old days when BA gave a toss about something other than the bottom line, BA recognised that CAP371 was a maximum, not a target. As such the BLR duty limits were set to what BA recognised to be a safer limit, not an easier limit. Short haul BLR limit are rarely more than 30 minutes shorter than scheme limits.
The point you have wholly failed to grasp, and the reason for my belief that you work at LGW, is that you cannot achieve the sort of hours Ryanair pilots get at LHR. Ryanair pilots spend the day on the aircraft, 25 minutes on the ground at each destination, they do 5 or 6 sectors then they go home. That is not possible at LHR. BA cannot reliably achieve even a 45 minute turnaround at LHR. Delays and disruption on the ground are rife. Aircraft changes are commonplace. Try looking at the ration of duty hours to flying hours in BA shorthaul. You'll find that 820 flying hours you'll be logging about 1500 duty hours. To achieve a 10% increase you are looking at least another 150 duty hours, probablyh more due to the inefficiency of short sector days. It is painfully simplistic to say "Oh they could just fly 10% more hours" and completely ignores the fact that people would start to run into all sorts of CAP371 duty limitations.