Personally, I’d like to see the investment in charging points etc. For most people, if they can afford a new car costing £20k, £30k, etc, £3.5k discount is not an insurmountable hurdle. What is a hurdle, if I look at my neck of the woods, is the distinct lack of public charging infrastructure.
If EVs become £3.5k more expensive overnight on 31st March which in turn slows their sales you bet that manufacturers will start throwing bigger discounts in to the equation.