The 40p milage allowance is only applicable to people who own the vehicle, so if you paid for the vehicle out of your own pocket and not through the business then you can claim the 40p allowance per mile. It costs approx 10p per mile to run a vehicle (dep on travels and engine etc) and the additional 30p is to cover tyres, insurance, mot, road tax, service etc etc
If you do a lot of traveling per week, say upwards of 300 miles per week the the 40p method mabie best as it will more than cover the fuel and insurances etc that you have paid out of your own pocket.
But you cant claim the 40p rate if any part of the vehicle is being paid through the business, insurance, fuel recipts, initial cost of vehicle etc. The vehicle must be owned and maintained by YOU.
Im not 100% but i think it possible to purchase your own vehicle through the business but an average market value price must only be paid and that will enable you to be able to put all costs through the business if the 40 rate method isnt cost effective for your business and/or you. But dont quote me on the above statement as im not 100% on that.
If you only do say for example 100 miles per week then it is doubtfull that the 40p rate method will be financially beneficial to yourself but it is easy to work out the maths and decide from there whats best for you