Vehicle Excise Duty doesn’t (generally) change retroactively, so 5 years ago they were telling whoever bought that diesel car to buy diesel.
The bands are now (since April 2017) flat regardless of emissions for petrol and diesel [1], so the encouragement is to buy either an electric car or a petrol/diesel car (as you prefer) with a new price less than £40k (or more than 5 years old).
The only part that changes with emissions now is the fee at registration, which is factored into the purchase price (for a new car) or already paid (for a used car).
The bands are now (since April 2017) flat regardless of emissions for petrol and diesel [1], so the encouragement is to buy either an electric car or a petrol/diesel car (as you prefer) with a new price less than £40k (or more than 5 years old).
The only part that changes with emissions now is the fee at registration, which is factored into the purchase price (for a new car) or already paid (for a used car).
1: https://www.gov.uk/vehicle-tax-rate-tables