A BA Holiday is made of a flight and another component. To change the flight will cost whatever change fee is involbved in the fare rules. Some of the flights may even be non-changeable entirely even though those are rare on BA (but not unheard of, especially, historically, for some sale fares). The only difference with just being the flight alone is that if you decide to cancel even a non-changeable flight, you will not lose more than the deposit (ie BA won't go after you to pay the full cost of the booked trip).

Same goes for hotels, they can be anything from completely non changeable to changeable for free or with a penalty.