15: London, England Most densely populated 1km sq: 20,477 people. Britain’s most densely populated square kilometre is in west London. It has just over 20,000 people living there – roughly a 10th of the most closely packed area in Dhaka, Bangladesh. Cardiff contains Wales’s densest sq km with 11,291 people; Edinburgh has Scotland’s with 11,069 and Belfast contains Northern Ireland’s, with 8,555.