I am VAT registered, (In truth, I should say 'my company is VAT registered' since it and I are separate entities) but it is far from simple to say that claiming back your fuel VAT will save you money
To do so you will become liable to be taxed for 'benefit-in-kind' which will cost you as much, or more, than you will save by the 16/17% reduction in your fuel bill.
On advice from my accountant I run the car privately and then claim mileage against the business and have done for the last twelve years, both as part of a partnership, and now, running a Limited Company.
Once registered you will need to charge your customers VAT on top of your normal charges, which could make you 20% more expensive than a non-registered competitor, reducing the volume of your business.
But a car forum isn't the best place to seek advice on taxation - I'd recommend you find yourself a good accountant at the very start so the business is set up correctly from Day One. It will save you a lot of hassle, and money, in the long run