If you really need a car for work, get them to give you a company car.
If you just need transport, drop £2-£3k on something OK. Don't take out a 10K loan at 17% interest to buy something that will lose most of its value before the loan's paid.
Putting big miles on your personal transport for work is a mug's game in my book. Yes, you do get expenses, and you can make a bit on them if you get good economy and aren't hit by too many repair bills. But all the stuff you have to do to a car has to be done twice as often, and it's a pain in the arse.
Whereas if it were a company car, you'd just stick it into their dealer every time something needed doing. And generally you'd be doing that on their time, not on your saturdays.