The dealer is not responsible for the Government increasing tax on the car in between you ordering the car and the car being delivered. The price was price locked. What happened was that the Showroom tax on the price increased. Nothing you can do about that unless you can somehow persuade the dealer to swallow the tax hike. But, in any case, it evens out. A car that would otherwise have been hit with an annual tax of £535 and a Showroom tax of £1120, now gets hit with a Showroom tax of £2000 and an annual tax of £140. So actually, after a couple of years, you're better off anyway. And your Mustang has better resale because, instead of being taxed at £535 a year, it's only taxed at £140 a year. I'm surprised the dealer has not explained that to you.