If it was my decision, I would make something like this:
0% until 31-3-2015.
From 2016 and onwards, step increments to normalise, possibly
10% until 31-3-2016, 20% until 31-3-2017 and so on, until it will be full FRT by 2025.
Since the budget doesn't run that far, I don't know whether or not it is possible.
This would work the way that whatever EV is imported, full FRT is calculated as pr current regulations, then the percentage is applied. If the FRT calculates to 130,000, that would mean for an EV, the FRT would be 13,000 (in that budget period where EV FRT is 10%), and so on. In other words, the FRT % is applied after the normal FRT is calculated.
Eventually, most cars will be electric, it's just a question of change of infrastructure, mindset, mass production advantage, battery technology and so on. It's here, and it's improving, and slowly, ICE cars will fade away as most carmakers will join the EV ride. Why? Because people will demand it.
It's all pointing in that direction - we are early movers, it is no longer "if" but "when".
A gradual increase will be fair for all, and give people time to buy the car they want. It will also speed up the process to implement it, as people will know "if we wait, the price will go up, let's do it NOW".
Hopefully, falling EV prices will match the gradual increase, so a 2025 Tesla Model S (or equivalent) will still cost the same as today, corrected for inflation.
Finally - had it been practically possible, I would even vote for a monthly increase, say
"0% FRT until 31-3-2015, after that, 1% increase each month until it reaches 100%"
It's not beneficial for anyone with larger steps, a gradual change is much better.