Im also a HGV driver and mine is also used as my daily drive leaving home at 4am most mornings. I have owned mine approx 18 months and have put nearly 22000 miles on it in that time. I get an average 21-22mpg and have managed 29mpg on a motorway run so 15mpg seems pretty awful. Tyres wise I have had the fronts on since buying the car and believe I will get 25k out of the fronts, is that a record?? Rears I have got between 8k-10k before having to replace. Touch would starts first time every time and has never let me down so I think you have just been unlucky. Its entrely up to you at the end of the day but to take such a loss if it were me I would keep running it until as you say you would have to scrap it.