BA have not ordered any aircraft, they never do. BA is just a subsidiary of a much larger organisation IAG. It is IAG who have signed an letter of intent to buy 200 B737 MAX aircraft, which will then be used by carious subsidiaries, one of which includes BA at LGW.
Admitedly interesting timing (means they were given an even better deal to announce now) and an interesting choice for a predominently Airbus shorthaul operator.