Intresting thread. Reminds me of an essay question on my IR course.
In order to answer the title question it is surely necessary to define the terms, i'd suggest "patriotism" and "bollocks" for starters.
There is a fine line between patriotism and nationalism IMO, and I'd define the former as the flag-waving, national-sport-team-supporting folk and the latter as the "we are superior because we come from X".
Assuming we are talking about a nebulous "proud of the country but not trying to force people to confirm and rejecting all others as undesirable" types, there is a lot to be said for "patriosm". Humans are social animals who function best in groups. The basic group would be a nuclear family, wider family, then ever increasing communities until you get to the (somewhat arbitrary, but in the cases of places like Britain and Australia pretty easy to define) geographical area called "country".
And yes, it is all accident of birth, but so is everything pretty much. So outside of accident of birth what ties you to a country? Moving elsewhere is a pretty massive move, and you do risk the chance of being a permanent outsider, for generations to come. And then the 2nd generation are often conflicted, especially where the immigrating generation really do go all out to show loyalty to their new home, but still retain strong ties to the old one. (and then you get idiots like Tebbit and his "cricket test")
In terms of generally supporting English/British teams in big events, yeah, why not? I was born here, they're selected from the best people with the same passport as me, and why not? It's arbitrary. Even if i don't care about indoor-ballooning, i'd probably "support" (ie be happy for them if they do well) the English/British team. I can get inter-British rivalry so long as it remains light-hearted and am not of the ABE mindset (notwithstanding: i'm actually English and i totally understand why Scottish people will always support England's opponent). In terms of support for sports teams (probably any other type of team) I'm usually: England, then NI, Wales, Scotland, Germany (I'm an immigrant), Ireland, European country, underdog. Not because i think they are best in any way, but they are the ones i feel closest affinity to.
For sport (again) i do notice that groups of nationals tend to be a bit over the top if they are watching their team in a different country. (singing a bit louder, embracing total strangers in the team shirt when a goal is scored etc)
And if you want to talk about feeling unable to display the national flag in their own country, speak to Germans who have only really started to embrace that since the world cup was held here. It is complicated.
the tl;dr version: it boils down to your reaction to the question "my country, right or wrong?"