“The plan for my own BEV is to pull out the legacy 12V battery & substitute a bank of LiFePO4 cells that I happen to have 80 of laying around unused.” Obviously you wouldn’t put these in a Motability car.
Obviously not, if the car kept breaking down because the 12V battery was flat you’d reject the car & choose a different one, after all it’s a clear design fault, even if the fix is trivially simple. So simple the MG ZS EV even has a button to recharge the 12V battery manually.
However, 4 x 15Ah cells to make the 12V, that would be a tiny but reliable battery that weighs about 1/4 the 12 lead acid battery, that means a little extra range, that’s why you’d do it to a motability car. Or just eat 1 less pie a month.