From what the dealers told me when I bought mine, most of their allocation has been sold (I think they get 4 per year) and build to order is a long wait, so I don't think their would be any to pre-register anyway.
I haven't seen many cars like the Cupra avaliable as pre-reg anyway as it tends to be the more mainstream models.
You might be able to save money and get an ex-demo though.