I think you're referring to the new Low Emission Zone Charge enforcement cameras. They don't monitor emissions, they are straightforward numberplate recognition cameras. They look up your numberplate and see if you have paid the LEZ fee. It applies to heavy goods vehicles and the like, and the idea seems to be that the more emissions your lorry puts out the more you are charged.
There is also a "fake" public consultation proposal on transport for London's site which basically talks about charging everyone depending on emissions, again looking up your numberplates. That doc talks about pre-2001 cars over 3l being classed as the highest band. It's not a real proposal though.
Nothing to do with cats or no cats though, even if it came in it would only be based on what was registered with the DVLA for your car when new.