"The reason he was happy to sing, possibly with gusto, the Red flag and not the national anthem is because he believes in the cause of the red flag but not in the nation and sovereignty. "
I'd say that that is fundamentally incorrect. I think Corbyn is perfectly "patriotic" in that sense. After all, he is anti-EU for the reason that he sees it as a "capitalist club" detrimental to the interests of British workers. Of course there is a big conflict there between being liberal on immigration and protecting "British" jobs which is why debate on the EU was, disgracefully, suppressed at the conference.
So it is a question of "what do you think iis best for the country?". I personally don't think obsessing about singing the National Anthem is worthwhile, but nor do I think the Red Flag is an apt for suitable anthem for anyone in the 21st century. As I said before, two conflicting mindsets, and both outdated.
Vince is your man :-)