Chessington is £60 plus per ticket nowadays. A typical family paying gate prices plus food/drink and parking or whatever would probably be spending £200/300+ for the day out.
If they are still driving a polluting, non-compliant vehicle I’d argue they can afford the £12.50 charge for a one day trip to Chessington.
Completely agree that rationally the additional cost is insignificant. But it's a psychological barrier that will stop some people coming, just as it stops many from entering the zone for other reasons when in reality £12.50 for a one-off visit is pretty trivial. A quick look on TripAdvisor shows reviews like this:





