After hearing from Citroën about their excluded cars, I tried to dig through the government's website and see which cars from other manufacturers are exempt. But to no avail. If anyone has the list of cars that would be exempt under the new rules, could you say so in the comments? This BBC article talks about the general exemptions, but doesn't break it down by model type. There's a lot more detail in this BBC article, but it's focused on parking fees. Is that what Citroën means? Their release is awfully unclear, but I thought they mean the daily driving charges. Under the proposed changes to the parking fees, Band A cars (basically electric vehicles) would park for free. Band B cars (like the Prius and the Renault Megane dCi 106 diesel) get a 50 percent reduction and the scale continues until gas guzzlers are paying 50 percent more to park in London.
[Update: Yes, Citroën is talking about daily driving charges.]