Windsheer
The last figures I saw suggested that in percentage terms both BRS and CWL take about 10% of their overall passenger numbers from the other's 'territory'.
So I don't think it is a case of BRS grabbing Welsh pax anymore than it is of CWL grabbing English ones.
There is a certain overlap in the two airports' catchment areas and to an extent they provide a complementary service for all airline travellers in South Wales/West of England.
For instance, I was talking to someone from Bristol the other day who is taking her family to Toronto from CWL with Zoom because no flight exists from BRS.