It's really not worth the hassle irrespective of how you feel about the gaff. It's gone the day you walk out the door for the last time, so just leave it.
If you had wanted to leverage your new job offer (and leaving) to change the climate
in the company, stay and get a pay rise - then the time to do it is informally in a meeting before you accept the other offer, so it's too late now. Beyond that let it go.
Letter from me is always short and sweet.
[Gaffer's name]
Please accept my resignation from my post as per the terms and conditions of my contract. i can confirm my final date of employment will be [insert date].
Your sincerely
As for the car, usually it's a win win, the tax is not so bad, I paid about £80 a month in tax for car benefit in kind. That was for a brand new 1.8 tdi astra sports hatch. I also got a company fuel card, and was a recipient of fuel benefit in kind, which was £40 quid a month for that vehicle, but meant unlimited private mileage. The upside is not tax, no service costs, no insurance (which is massive at moment). Bigger the car, more you pay, if you are high rate taxpayer, more you pay.
Also, hmrc have a long memory, if some cock in your works finance department screws up your deductions, the tax man will claw it back, so keep an eye out for the p60d your company submits for you - if its wrong fix it them, not 3 years later when hmrc track it.
Chances are you will not be awarded no-claims whilst you are in the schema, this could bite when you come out. I got hit hard by this when i left the scheme, as I could not find an insurer who would award credit for that period and was still cheaper than my best quote with no continuous no claims awared (I've never had an accident....£800 comp).