Here’s a list of differences between British and American English like the ones mentioned so far:
It’s really pretty hopeless to explain why any particular difference happens. There’s no general rule about how the differences work. There’s no overall explanation about why they happened in one country and not in another. Just learn them as you go along and don’t try to create big theories about why they exist.
I have never thought of “in hospital” being in any way odd, as that’s how I’ve always said it, but now that I look at it more objectively, it is a strange phrasing!
Nothing at all strange about it; he’s in court, she’s in college, I’m in hospital, you’re in prison, we’re in school. No particular court, college, hospital, prison or school is implied. He’s in the court would mean a particular court already referred to, or alternatively would mean there was only one possible court that he could be in.
And, to my ear, the same goes for “hospital”. I’m in the hospital implies that I’m in a particular hospital, and furthermore that it’s somehow pertinent to the conversation that I’m in that hospital, and not another.
There is a prevailing myth that whereas in the USA, people say “waiting in line”, in Britain it’s (supposedly) “waiting on line”. Except this is utterly and completely false. Nobody says it, but the myth won’t go away.
In Britain, it’s “waiting in a queue” or “queueing”, or (rarely) “waiting in line”.
In another current thread on GQ, about “The? Ukraine”, it is mentioned that Russian, and other Slavonic languages, simply do not have the definite or indefinite article. Reading this present thread, one might suspect that those Slavs are perhaps on to something there – non-existence of the pestiferous “the”, could save a lot of annoyance and contention.