What are the symbols of national unity?

I have a reasonable familiarity with thee countries. UK is the country of my birth, passport and identity, (though once upon a time I would have identified myself as British I would now say English. I will not say the reasons for the change here but will answer if you ask me why.) The symbol of British unity is Her Majesty, the Queen, of whom I am a loyal subject. I identify as a British subject, not a European citizen  (but that is another story). Most of my fellow Brits identify as supporters of our monarchy. Republicans are a small minority but over-represented in the media, especially the BBC and C4.

OTH, in the USA the flag is the symbol and the anthem. In the UK, with recent devolution, the Union flag is not honoured as it used to be, especially in Scotland. In England, St George is now flown from other places than parish churches but it is often maligned by the Left as a symbol of the  (far) Right. The far right may use it but we English own it. But flag flying outside one's home is rare in UK, common in America. Flag burning is pretty well unknown in UK except by a few extremist Islamists who do not really identify with UK at all - except perhaps for its benefits system Americans by and large take their anthem seriously. In the UK the national anthem used to be played in cinemas and theatres and audiences stood. No more. In sporting events with four different UK teams the Scots and Welsh now have their own anthems and may boo the national anthem as being English. The English have no other anthem than 'God Save The Queen', though others have been suggested.

I have some present concern at the political polarisation in USA so that the presidency seems to no longer unite the nation. Republicans have shown such distaste for Obama and now Democrats will show even mor contempt for Trump. The president no longer unites the nation by reason of his office. OTH in UK, even republicans usually have great respect for the Queen as a person, for her integrity and service.

Nigeria where we lived for 12 years definitely has a unity problem. We moved there 9 months after the end of a four year civil war and, by and large the victors were magnanimous and that was for the good of the country, though in the former Biafra, resentment and secessionism are not dead. More important is the history. Why Nigeria? It is the century old product of the British uniting former smaller colonies there into one. But apart from (colonial) history, the flag and one national(foreign) language, English, what unites Nigeria? Go from south to north and there are different dominant religions and languages, different major and minor ethnicities, different climate, flora and fauna What can be the future uniting factor and something more than the symbolic national flag? The major rift is between Islam and Christianity, both imported religions but zealously followed. Both claiming to be universal and exclusively true faiths. And one having extremists who want to remove all Western influences, religious and secular, and are not it seems concerned if they kill more of their
co-religionists than followers the other major faith which they hate.