Why do Brits say IN hospital instead of THE and which is correct?

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.