There is a tendency these days for people to use the word "which" in place of "that". This is another grammatical mistake that comes about because schools are not teaching sentence structure. Some of the worst offenders are BBC news reporters and announcers. Is it also more prevalent in the the South of England? It does seem to me to be the case. I don't know whether or not the people who make this mistake in speech make the same mistake when writing.
The word "which" is used after a comma to start a phrase or a clause that is supplementary to the sentence. The phrase or clause also ends with a comma. If the whole phrase or clause is removed from the sentence, the sentence still makes sense.
An example: "The house, which is soon to be demolished, is unoccupied." This is referring to a specific house.
The word "that" is used to start a phrase or clause to complete a description of a noun. There is no comma before "that". If the phrase or clause that follows "that" is removed, the sentence is broken and makes no sense.
An example: "The house that is to be demolished is unoccupied." This also refers to a specific house but implies that there are occupied houses that are to remain.
This is incorrect: "The house which is to be demolished is unoccupied."
This is incorrect: "The house, that is to be demolished, is unoccupied."
