Is "that' in a title capitalised?

It would depend upon what part of speech "that" is being used as--it can be a conjunction, and adverb, an adjective. a pronoun. Without a specific example, it would be impossible to say. Of course, a good rule of thumb for all titles is that the first and last words of the title are capitalized, no matter the part of speech.

As I believe I have mentioned before, this does not vary from American usage to British usage.

Thanks, Setanta.

Film title:

The House That Jack Built

Thanks.