What is the 'optimal' taper ratio?
The excerpt you posted mentions a further modification of the Oswald efficiency/taper ratio curve which is not applied to the graph in the excerpt. With that modification (which I believe is included in the following pages of the book), the optimum point may move to 0,45. For an assignment, the other places you found 0,45 from and the BAe wings are sufficient motivations to set it at 0,45 IMHO.
I'd like to mention that taper ratio is not the only way to adjust spanwise lift distribution, you can change the airfoils (which you mentioned you have excluded from your design) or you can add twist to the wing so that the wingtips work at a lower AoA. This has the added benefit of making the wing root stall before the wingtips where the ailerons are located.