The only reason why Nicholls may have been used, is Whomes didn't want his car seen within the area on the night. The motorways would have CCTV. He would then not be able to claim he was at home that night.
The solution to this would be to use a car he has no connection to. He was going to kill 3 people, so obtaining such a vehicle for one night would not be hard.
However even if Whomes's car was spotted by CCTV on the night, a long way from the massacre sight, it wouldn't be enough for the police.
So still no reason to use Nicholl's.