Jingoism comes from the quote - "We don't want to fight, but by Jingo, if we do . . . "
Having a national flag on view doesn't have to be picking a fight.
For instance -
- when delegates are are an international conference, and the relevant flag is an instant identifier.
- when athletes are competing internationally (for the same reason, and to give a focus for the euphoria when/if they win)
- when the PM or another minister is giving a news conference of national importance. ("Speaking to the nation")
- in the chamber of the HoC (reminding MPs and cabinet ministers that they are supposed to be debating and enacting legislature on behalf of the whole populations of the United Kingdom)
- on goods manufactured in the UK (not just packed there) so long as they are of a high standard. A reputation for maintaining quality is a useful asset, particularly when you are trying to establish a whole raft of new trade deals.
Making it compulsory defeats the purpose - it could become something to resist because it is imposed. They never learn, do they? If you demand love, you get resentment.
However, I thought Government buildings already flew the flag.