In Hospital vs. At the Hospital: Understanding British English Usage

The correct way to say it is "in hospital".

The reason why Brits say "in hospital" instead of "the hospital" is because the word "hospital" is a mass noun in British English. This means that it refers to a general concept rather than a specific instance of that concept. For example, you would say "I'm going to the hospital" if you were referring to a specific hospital, but you would say "I'm in hospital" if you were referring to the general concept of being in a hospital.

Using the definite article "the" with a mass noun is considered incorrect in British English. So, while "in hospital" is the correct way to say it, "in the hospital" is not.