Just to clarify:
Those contractors/tradesmen driving older vans who currently pay the Congestion Charge, will also be already paying the ULEZ charge anyway (because the Congestion Charge zone is all ULEZ, has been for some time now).
Those contractors/tradesmen who only started paying the ULEZ charge now, won't be paying the Congestion Charge, because the new ULEZ is outside of the Congestion Charge zone.
So, the contractors/tradesmen will wither already be used to paying ULEZ plus CC, or they will start to pay ULEZ now but not CC.