Contractors won't sue RBS or anyone else over this, because they would lose. All that money on expensive QC's (also contractors) would be wasted.
Hirers of contractors are free to pay whatever rate they want, so if contractors (who claim to run their own businesses) don't like it, then look elsewhere.
All this complaining is starting to sound like permanent staff who actually are covered by employment laws - but contractors aren't, because they are not permanent. You can't have it both ways.
I suspect most of the complaining is coming from "contractors" who are de facto permanent staff, but have arranged recompense to avoid paying their fair share of tax.