I always wonder if those home delivery fast food outlets rip people off by making them use their own cars to do the deliveries. Firstly what sort of wage would you expect to earn in any normal job. if it is way lower that 9 pounds an hour then maybe the extra money is enough to warrant having to supply your own car. if 9 pounds is the normal working wage there then you are being ripped off as the pizza place is getting free use of your car.
If you take on the job record all the car expenses and the mileage you do each shift you do so you can claim the car usage off your taxation and get a tax refund for it. Keep a log book with the start mileage for the shift and then record the mileage at the end of the shift. You should be able to claim back the percentage of car usage off your taxable income including a percentage of the fuel, tyres, mechanical work, car servicing costs, registration and insurance costs, and likely a percentage of your auto association fees, etc. But you must keep full records or they will not believe you and you will get no tax refund for these expenses. Get receipts for all fuel you buy for the car, as well as for all parts and work you get done on the car as proof of your claims or the tax department will not allow your claim.
Also keep a record of earnings although I would not mention the tips as some of these places likely pay cash in hand and if so they are not taking out tax. If you want to risk this and accept the untaxed money then hopefully that will help with the car costs through the extra you get through no tax being taken out but of course then you cannot claim the car running costs off your tax as you paid no tax. Remember you take a risk if they offer cash in hand with no tax taken out as if they are investigated and the tax dept. discovers you have not been paying tax on your earnings you risk fines as well as paying the tax money out of your own pocket. Buy a good quality notebook and record all this information neatly so it can easily be read by a tax agent. Even an excel worksheet all laid out neatly will make it easy to add up costs and profits so it is easy to see if the job is paying well enough and make it easier at tax time. Make sure you back it up on a memory stick in case your computer dies.
Only you can work out if it is a financially viable job by working how much below 9 pounds the wages should be and what it leaves you for car running expenses.