Not impossible, for home charging, a smartmeter on the charger port for billing the house, for public charging, the taxation would be added to the bill at source
OK, what's the tax rate for overnight charging? If someone is paying 6p per kWh and the tax is 100% that's still 12p.
Even at 3 miles per kWh that's 4p a mile, and so the total cost would still be £480 rather than (pre war) £1,900 for someone doing 12,000 miles a year.
That's still around £120 a month in savings (bearing in mind my lease is £200 for 12,000 miles a year) so whilst the maths isn't so clear cut, it's still a fairly significant saving.
Also, whist we have a smart charger, we also have a 13amp socket (external, but for others that could be within a garage), we can charge from that, even if that meant paying for 3kWh at 30p per kWh (£0.90) and a further 22kWh at 6p (£1.32) that's still cheaper (£2.22) than paying 12p through the charger (£3). Whist that's only £80 a year in savings, people will do it.
That's assuming we don't have a battery system and charge that overnight and use that (around 7p per kWh once you allow for losses) for the bit of the charge we didn't do because we flicked the switch to change the car before the off peak rate started as we didn't want to wait for it and went to bed and didn't pay for a timer to be installed at the interal switch (between the fuseboard and external socket).
Either of which could reduce the extra cost down by a further £75 a year.
If we pay for a timer switch we'd buy one with app control (which isn't much more), so when at home we'd use it to draw power from our solar (rather than exporting) as whist that "losses" us 12p in export income we'd also be no worse off and better off if the tax was set higher.
That all assumes we're not using a variable rate based on energy supply where we could use the app to charge the car when the rate is negative. Then what's the tax rate?
As then as we are paying zero a 100% rate is still going to be zero. If it's a fixed value (say 6p) as long as the rate is lower than +4p then even using the smart charger during the day would be slightly cheaper, let alone using our WiFi controlled charger.
Bottom line, my extra tax bill could be £0 and my electric bill maybe £5 (due to battery losses) but maybe not a lot. Maybe up to £150 to pay an electrician to install that WiFi timer switch (but only if we did that at a time when we weren't requiring them for something else) but that "pays" for itself within (at most) 2 years from the savings (with negative pricing it could be much faster).
Nothing I've suggested is likely to ever be illegal in terms trying to limit my tax bill and HMRC make almost nothing from me for home charging.
I doubt that I'll be the only one who could do that. However, again the people who would end up not being able to limit their tax bill will be those who live in places where they have to use a bollard smart charger (flats, terraced houses, etc.) and so again those who have money (i.e. can afford a driveway or a garage with power and solar systems) will pay the least tax, when they could most afford to pay it.