score:2
Yes, if booked as one through ticket, declining to take the first leg usually results in the second leg being cancelled on BA. [Indeed, this is not reliably the case (some friends did an experiment recently to test this), but unless you are willing to front the cost of a walk up ticket to satiate your curiosity, I advise calling ahead.]
If booked after 28 April, domestic add ons are not longer free and you will get 4500 Avios back. If booked before, domestic add ons were free at this time and no Avios refund will be due.
If you hold Gold Guest List status with BA, the change is free. Otherwise, there is a £35 change fee. http://www.britishairways.com/en-gb/executive-club/spending-avios/booking-and-service-fees