With respect, that would ensure them no work at all with the BBC. They are very far from union friendly if it affects them.
The BBC have always had a 'very small cheques tartan' policy for decades. The way it works, is they will employ an act for £xx, then when the act is rebooked, and asks for £yy after knocking his pipe out getting well known, the answer is you did it for £xx last time, you can do it for that or we'll get somebody else.
The only way you can break this, is by pulling an audience that they cannot argue against.
Here's a good example Paul Jones used to name his own fee for the Blues show due to his position in the music world when there were other musicians who were better broadcasters. You see it again and again, fame beats talent in broadcasting with the BBC. Look what happened to Mike Harding. Union representation will see the door slammed very loudly. Sorry.