The OP's link states that, since 2019, some 4,000 ex racehorses have been sent to the abattoir - most of these from Ireland - where ill-treatment is apparently carried out.
My link states that, as at 2018, some 13,000 ex racehorses were registered with the BHA to be retrained in other aspects of equestrian sports. "British racing's owners - represented by the Racecourse Owners Association - have recently agreed to further increase their prize money contribution to funding for rehoming and retraining of retired racehorses."
The BHA are ever-mindful of the opposition to horseracing and do their utmost to ensure the safety of the animals during their racing careers and afterward.
My guess is that the first the BHA knew of the apparent 'ill-treatment' of this abattoir, is when they were approached for a response by the Panorama team. They will, no doubt, take the necessary action to ensure that this practice ends forthwith.