I really don't see what Corbyn not singing the National Anthem has to do with anything. He's a republican. So are a great many UK nationals. There are even a few on Gnet, I've noticed. It's not actually illegal to be a republican. It is perfectly possible to be a republican and wish to work towards the abolition of the monarchy within the framework of our constitution (remember, Parliament is sovereign and can make any law it likes so long as it has the support of the House).
What is more, if Prince Harry is correct none of the 'Royals' are particularly keen to take on the job, anyway (though I suspect he's not speaking for his dad) so perhaps they're quite relieved at the possibility of a republican in government...
And republicans don't particularly want God to save our queen to rule over us.. it's a bit hypocritical to sing the National Anthem if you feel like that..