99.9% of the time it’ll be the original registration but it’s not guaranteed, I always stick the original plates in the boot and only once in 30 years they weren’t the ones to go back on.
I phoned motability in December to swap my plate back and they done it there and then over the phone, I got an instant email with all the updated paperwork so took no longer than 30 minutes. I had expected it to take longer so phoned 6 weeks before changeover in case of any Xmas delays.
The garage can arrange it all for you and I’ve done this in the past with private sales but never with motability…. don’t trust them 100% yet ?♂️