TBH fuel costs are the least of your worries.
When it was new, in the UK, a Supra was a £40k car with £40k car running costs. The purchase price has dropped like a stone, but the spare parts remain at their previous levels
I'm not trying to be flash here, but if you've really got to ask, you probably can't afford to run one. Sure you'll probably afford the 18 or so mpg, but when parts need replacing you'll really know what running a performance car is all about
If you're actually doing a high mileage, then unless you're in a very well paid job, a Supra probably isn't the best choice for a daily driver.
Just a quick calculation on your figures, (using 18 mpg and fuel at £0.95 /litre) shows that out of your 40p per mile allowance, virtually 24p of that will go on petrol. By the time you've taken Insurance, Tax, MoT and servicing costs into account, you're going to be subsidising your company for every mile that you drive