Source:  The Flag Institute

Half-mast means the flag is flown two-thirds of the way up the flagpole, with at least the height of the flag between the top of the flag and the top of the flagpole. Flags cannot be flown at half-mast on poles that are more than 45° from the vertical, but a mourning cravat can be used instead (see below).

When a flag is to be flown at half-mast, it should first be raised all the way to the top of the mast, allowed to remain there for a second and then be lowered to the half-mast position. When it is being lowered from half-mast, it should again be raised to the top of the mast for a second before being fully lowered.

When a British national flag is at half-mast, other flags on the same stand of poles should also be at half-mast or should not be flown at all. Flags of foreign nations should not be flown, unless their country is also observing mourning.

The Royal Standard never flies at half-mast. It represents the Monarchy, which is continuous, and it would therefore be inappropriate for it to fly at half-mast.

Flags should be flown at half-mast on the following occasions:

1. From the announcement of the death until the funeral of the Sovereign, except on Proclamation Day when flags are flown at full-mast following the proclamation.
2. From the announcement of the death until the funeral of a member of the Royal Family styled ‘Royal Highness’, subject to special commands from the Sovereign in each case.
3. On the day of the announcement of the death and on the day of the funeral of other members of the Royal Family, subject to special commands from the Sovereign in each case.
4. The funerals of foreign Rulers, subject to special commands from the Sovereign in each case.
5. The funerals of Prime Ministers and ex-Prime Ministers of the United Kingdom, subject to special commands from the Sovereign in each case.
6. The funerals of First Ministers and ex-First Ministers of Scotland, Wales and Northern Ireland, subject to special commands from the Sovereign in each case. Unless otherwise commanded by the Sovereign, this only applies to flags in their respective countries.
7. At British Embassies, High Commissions and Missions when flags in the host country are flown at half-mast, subject to the discretion of the Chef de Mission.
8. Any other occasions where the Sovereign has given a special command.

If the body of a very distinguished citizen is lying in a building, the flag should fly at half-mast on that building until the body has left.

The above covers Royal and National Mourning, but flags may be flown at half-mast on private or non-Government buildings on other relevant occasions. Flags fly at full-mast on Remembrance Sunday.