London is, indisputably, the capital of Britain’s royal life. Here, the young Princess Elizabeth grew up before the Second World War. Here, she and her sister, Margaret, mingled, incognito, with the joyous crowds on VE Day. It is still the centre of a working monarchy. Trevor McDonald reports on the tradition of Changing the Guard at Buckingham Palace and visits Horseguards to meet the man who is in charge of the troops who guard The Queen.