Get a cheap £1k car & always keep a look out for a 2nd one at a bargain price. So when car 1 fails, you can hop straight into Car 2.
If you getting the mileage money your allowed (45p upto 10k miles, then 25p afterwards) - on 30k miles a year your looking £9500.
Dont forget it resets in April - Prob spent half of that on fuel (35-40mpg) - left with £4750 to buy, tax, insure & maintain your vehicle per year.
Tax - £250 (worst case scenario), Insurance about £1500 (WCS) left with £3k to buy & maintain a car - dont forget your wage slip will go up prob by about £150 a month as you will no longer pay P11D company car benefit. You could get a loan & not be worse off - or even buy from a car supermarket if your in a hurry.
I would just buy a cheap thing & run it into the ground. My stepdad drives 150miles a day round trip & he uses a rover 800. Tends to last him 3-4 years without any major failures. Proper old mans car, but great for motorway driving - IIRC your in the SouthWest - how about :-
http://www.ebay.co.uk/itm/Rover-820-2-0i-SLi-16v-This-car-is-out-of-the-box-condition-/110923486377?pt=Automobiles_UK&hash=item19d38e14a9If your not getting the full mileage, then speak to the Tax man as you can offset it against your Tax.