When you booked you would have booked each car, not person, with its registration number. As long as you turn up with these cars you'll be OK, but if you have changed any cars go on their site and amend the booking, otherwise the check in takes longer.
I never give the registration number, in case we decided to take an alternative vehicle. Never presents a problem. On the Outbound, all I have to do is enter the booking ref, on their slow and very poor touch screens, and on the return it recognises the car and pulls up the booking.
Provided the vehicle falls in to the same category you are fine to just turn up with a different reg.
The only issue is that you will need to enter your booking ref in to the entry barrier, as the number plate recognition won't be able to find your booking.