you will only be able to claim:
40p for the 1st 10,000 business miles travelled and 25p per mile thereafter as 'allowable expenses' as you wil be using your own car for work.

But:

You will be incurring all the costs, servicing, depreciation etc etc.

If you are going to lease, I'd be looking at a deal that includes all the vehicle maintenance of the period of the lease. the monthly lease may be slightly more expensive, but it'll be worh it as you wont be paying for servicing etc.

IMHO having a company car is always a better option than a 'cash allowance' in your wages as an alternative and being told to get your own.
EDIT: to explain the above point:

company provides a car worth 'say' 20K and the car has a Co2 rating that put it in the 20% tax bracket, so the 'taxable benefit' to the employee is 20% of £20K = £4000.
The employee is a 20% taxpayer, so the tax payable on the benefit is £4000 x 20% = £800 per year in tax. So that employee has £20K worth of car on his/her drive for £800 per year!.

contrast that with the company giving the employee an extra £500 per month in their wages instead of a company car. the £500 is taxed via the pay packet at 20%, so again if the employee is a 20% taxpayer £500 x 20% = £100, so after tax, the employee has £400 net to go out and buy a car, tax it, insure it, run it for business etc etc...not quite a good deal is it!!!.....

Also need to take into account the security of your job. there have been horror stories of companies who have gone down this route, the employeers have signed themselves up to a 3 year 'options' type deal for a car and then the company has gone bust, leaving the ex employees on a personal contract lease they cant pay for........
If you had a company car and the company went under, you'd just have to give the car back....

just food for thought........