Just booked
Was going on the tube now I am driving^
It appears that you can amend the bookings as I just done a bit of time changing & it allowed it, I also went along the route of seeing if it would cancel but lost my nerve near the end but it did appear to let me go most of the way but I was worried it may cancel & no refund!!

Also on the BA miles on Holiday extras I have used them about 4 times & all the miles have posted within about three weeks of me collecting the car.