Hi Horseworth. Money from selling your own car, even a mid-life crisis indulgence car, doesn't count as income as far as the tax man (or woman) is concerned, even if it's worth more now than you paid for it.
If you were to start buying cars with the intention of selling them on for a profit that's a different matter. You'd be required to declare that income and be taxed on it. The rate would depend on what tax bracket you're already in and whether the income would tip you over the limit for the next bracket.
There's an urban myth that you can legally buy and sell up to six cars a year for profit without paying any tax but that simply isn't true. HMRC have now started monitoring activity on ebay and other m/v outlets to see if people are running undeclared businesses, and there will be people faced with some serious tax bills when the tax man comes a knocking.
Good luck selling the TF and finding something a bit more individual and exciting to replace it.