the Battle of Britain = 不列颠战役

decision = 结论、结果  a report explaining why a legal or official judgment was made in a particular way

turn on/upon sth. = depend on/upon 取决于

on which decision the Battle of Britain would turn

which的先行词为the exchange rate in fighters

on which decision = on the desicion of the exchange rate in fighters

the Battle of Britain would turn on (depend on) the decision of the exchange rate in fighters

不列颠战役将取决于英德两国战斗机损失的比率。