It would be perfectly grammatical, I believe, even in BrE (I speak AmE), to say
I just know he is lying in a hospital somewhere. He doesn't know how to control that motorcycle very well yet.
The reference there is to an unknown but particular hospital, but God knows where.
In BrE, when you're referring to his locative status (he is "hospitalized"), and not to a specific hospital, you do not use an article:
He's laid up in hospital.
In AmE, when referring to no hospital in particular, but to the status "hospitalized", instead of using the zero article you would use the definite article:
He's laid up in the hospital.
We use the definite article to refer to the place in general terms, to say that he has been hospitalized.