Why not just a fixed increase the VED on EV (£300), PHEV (£150)and possibly Hybrids (£100)
Because it's not proportionate to mileage-driven and economy (like fuel duty is today). With fuel duty, the more you drive the more you pay, AND the less efficient the more you pay.
£300 for a full EV is fine if you are targeting 10,000 miles a year. But if you only do 2000, it's a massive hike.
We have 3 EVs in the house hold - one big one that does about 12 to 15K a year, and two small ones, both much lighter and more efficient, that do that 2000 mile number.
PHEVs are hard to deal with because of the mix of fuel use. My son charges his every night, and gets about 20 to 25 miles range from it, commuting, so about 4000 miles. Any additional mileage on that, he is already paying duty.
Plain hybrids are trying to be efficient (better MPG) and pay all their mileage in VED. If you are going to charge them more, you have to raise take for all non-plug in vehicles.