I was hoping not to need to come back on this.
BCR is an economic/management tool. As its name implies it is based on estimating future benefits and costs, sometimes a long way into the future. The recent "cost of living crisis" has demonstrated that top economists in the Bank of England and the Office of Budget responsibility were unable to make good forecasts of inflation and interest rates 2 years ahead, never mind 20 years. BCR depends hugely on assumptions about future inflation and interest rates. Recent experience should teach us that no BCR calculation is ever correct, it is just an estimate based on a particular set of assumptions about future benefits and costs.
At this point in time, use of BCR is included in HM Treasury guidance. It is the correct process to follow, in order to stay within the current guidance, but that doesn't make it the correct process on a broader perspective. There are different tools in the box, and different processes that could be followed. There are no laws, statute or economic, that say that BCR must be used. The current guidelines are the choice of the current Chancellor of the Exchequer, following (or not!) the advice of HM Treasury civil servants. If the Chancellor of the Exchequer decided, that guidance could be withdrawn tomorrow, and new guidance could replace it on Monday morning.