Is it permissible for a Muslim to become a police officer in London?

Answer

It is not permissible to become a police officer in London nor any where else in the world, even in Muslim countries. All government and security institutions are agencies of zulm (oppression and injustice). Governments in all countries function in accordance with the law of Taaghoot. All laws besides Allah's Law are haraam and kufr. It is haraam to aid and abet any institution of kufr.

Some centuries ago a tailor used to sew the garments for the Sultan of the time. This tailor suffering a pang of adverse conscience because he knew that the Sultan was a zaalim (tyrant/ oppressor) asked a Faqeeh if sewing garments for the Sultan was aiding him in zulm. The Faqeeh responded: "It is not only aiding zulm. It is zulm per se."