I think you are rather missing my point, here. The body armour that Muggle Britain would invent would not be the kind available to Harry as a desperate improvisation. In a year or two, after a crash research program, Muggle Britain would understand how magic works, what materials block it, and would mass-produce the resulting body armour that does block curses up to and including Avada Kedavra. It would be based on tinfoil because everyone knows that tinfoil hats block the government's mind-controlling rays.