No, hold booking for a fee doesn’t apply to holidays bookings and you can’t amend any aspect of it (flight times/dates etc even if the same price was available when you first looked) when you go to confirm it as it’s no longer the booking you held. The only thing you can change is names where there are multiple passengers but you haven’t entered them at the point of holding the booking. They appear as the lead passenger’s full name then others as LASTNAME/TWO, LASTNAME/THREE so these must be updated. Reducing the cost using Avios also doesn’t apply to this type of booking.

You'd need to make a choice between holding the flights and booking car hire separately or taking a risk on whether the package price goes up. There is a 24 hour cooling off period for packages booked over the phone though, I believe.