I read it also and there was something about having a modified show car too . Can't remember exactly what it said will dig it out.
Discount for showman's vehicles
Some showman's vehicles are eligible for a 100% discount from the ULEZ daily charge if they are registered to a person following the business of a travelling showman and have been modified or specially constructed. Trailers and semi-trailers which have been modified or specially constructed are not eligible for the 100% discount.
For a vehicle to be eligible for the discount, modifications or special constructions for the purpose of the show must be either:
Used during the performance, orUsed for the purpose of providing the performance, orUsed for carrying performance equipment, orUsed for displaying animals to the public, orBelow the chassis level, such that it isn't possible to fit emission abatement equipment
If your vehicle meets the above criteria, you need to register with us to qualify for the discount. (Vehicles that are already registered with TfL for a LEZ showman's discount do not need to be registered again for ULEZ). This applies to both UK and non-UK vehicles.
If you receive any other discounts or exemptions for the Congestion Charge you still need to pay the ULEZ charge if you drive within the zone in a vehicle that does not meet the ULEZ standards.
But this is more a business vehicle for fairs and such I believe