I did the same stellar mileages prior to Motability, and used an independent local firm, always brilliant service and they must have liked me as we had 3 cars, using 12 tyres a year on average.
Better prices too. But clearly MB need a national chain to ensure everyone can be served wherever they are, so their choices are somewhat limited.
In life, it's not who you know that's important, it's how your wife found out.