@SlipperyDippery
It's mostly only English that do this. Scotland boo GSTQ but that's due to the nature of the song (crushing Scots)
So it’s ok for the Scots to boo the English anthem but not ok for the English to boo other anthems.
Which lyrics are about crushing Scotland?
There's a myth that the Scots like to perpetuate that this verse is still sang:
Lord, grant that Marshal Wade,
May by thy mighty aid,
Victory bring.
May he sedition hush,
and like a torrent rush,
Rebellious Scots to crush,
God save the King.
It was - for a while - during the Jacobite rebellion, but ceased to be used even before the song became our official National Anthem. The other side of the coin that Scots neglect to mention is that there was a pro-Jacobite verse:
God bless the prince, I pray,
God bless the prince, I pray,
Charlie I mean;
That Scotland we may see
Freed from vile Presbyt'ry,
Both George and his Feckie,
Ever so, Amen.
It's a weak attempt to justify the Scots behaviour and vilify the English.
Bottom line, because the Brits are more alike than they wish to admit.