How the Individual Vehicle Approval (IVA) scheme works if you’re making or importing a single vehicle or a very small number of vehicles.
Below is from TFL for background
Currently, only vehicles that meet Euro 6 standards (petrol and diesel), that emit no more than 75g/km of CO2 and have a minimum 20 mile zero emission capable range will qualify for the 100% cleaner vehicle discount.
From 25 October 2021, the cleaner vehicle discount will change so that only battery electric vehicles are eligible. Then from 25 December 2025, the cleaner vehicle discount will be discontinued. From this date, all vehicle owners, unless in receipt of another discount or exemption, will need to pay to enter the Congestion Charge zone during charging hours.