The St. George’s Cross is flown on Government buildings with two or more flag poles on 23 April alongside the Union Flag to mark St. George’s day.
During Trooping the Colour, only flags of Commonwealth countries are flown in Horse Guards Road. As explained in the answer of 8 January 2007, Official Report, column 263W, the Crown Dependencies and Overseas Territories are not member states of the Commonwealth so their flags are not flown.
The Union Flag is flown on Government buildings to mark specific national events including Remembrance day, Europe day, St. George’s day, Her Majesty’s Accession and Wedding day and birthdays of members of the royal family undertaking official duties.
There are currently no plans to change this.