There is nothing BRS can do about this in all honesty. The devolving of tax reducing powers is nothing to do with law/jurisdictions etc, just a simple fact of devolving Wales from Westminster for a more local approach, to help solve local problems.
We could argue that BRS has distorted the South Wales market over the last decade with EZY and FR, attracting so many over the bridge, so they cannot moan about CWL potentially being more attractive to long haul operators. But, obviously, they are going to moan, because at the end of the day they are potentially losing business. Simple, stop incentivising South Wales agents to switch sell BRS over CWL, to make it a fair playing field !