I tried this and they weren’t having it at all.
Couldn’t really be arsed to go all the way to the ombudsman as to be honest there was a car I wanted on the scheme anyway I just didn’t want to order right there and then.
Seems ridiculous to be honest because you could very easily game the system anyway by ordering a car with a very long lead time and cancelling before delivery as you get to keep the existing car during that period but it would be slightly screwing over the dealer.
In the end I took a 4 month extension (all they’d give me after arguing the toss) and have ordered a car likely to take another 5+ months further to that. (Ioniq 5 N Line S)