1. These are the books which you sent to me.
2. These are the books that you sent to me.
Am I right to say that "which/that you sent to me" is a restrictive (=defining) relative clause? Is it also true that In British English, both relative pronouns ("which", "that") can be interchangeably used in such sentence structures; but in formal American English only 'that" is preferred?
Can I say that, because it is a restrictive clause, I cannot insert a comma before "which"?
1. These are the books, which you send to me.