On the outbreak of war, the government had been faced with the question of what to do with non-British residents dubbed "enemy aliens," especially having regard to the fact that so many Germans and Austrians in the UK were Jewish refugees from Nazism.
A similar thing had happened during the First World War, when German and Austrian nationals were interned. It is certainly true that enemy activity was taking place, that spies and enemy agents were operating in Britain, however many people felt that the locking up of foreigners was an unnecessary step to take in a country supposedly fighting to free Europe from the grip of Nazism.