Be calm people. Perhaps it's the heat.

First, you don't have to use this form to cancel a booking, you can choose to use it or, as the OP has done, ring up and inquire about the refund figure before deciding whether to go ahead.

Second, some refunds are complicated to calculate, particularly if a sector has been flown (although the form suggests that it should not be used in such situations). Of course, a robust IT system plugged into all the required information might be able to do this, but this is BA.

Third, the wording on the form is not ideal, but this is BA. The attempt to impose, by some sort of variation, a term that the calculation of the refund amount is non-negotiable is almost certainly not enforceable (lacks consideration). The sum due by way of refund is ascertainable by application of the terms and conditions applicable to the booking. It is not arbitrary or discretionary. If BA gets that calculation wrong, then the figure is "negotiable" (ultimately all the way to a civil claim).