Judy795 wrote as follows: "You are allowed a large number of days layover in London, coming or going to your next destination (Paris). Yes, you need to call BA to do this ...".

99.9 percent of the readers of this post would conclude that Judy is telling us that a phone call is necessary to book a layover. The phrase "to do this" is clearly referring to the activity in the immediately prior sentence, namely, booking a layover. I posted merely to correct that incorrect advice, hoping to save those readers (and the OP) an unnecessary phone call and an unnecessary booking fee. I stand by my advice.

To get technical, this is a textbook application of the rule of grammatical construction known as "the last antecedent rule." The pronoun "this" must be interpreted as referencing the immediately preceding noun "layover."