Pre-Registration, Registration and Spelling
We're using the "pre-registration" term because:
a) It's in advance of the event and therefore not an on-site "registration".
b) When people arrive at the event itself, they're directed to the "registration" tent to either check-in (if they've "pre-registered") or pay the on-the-day fee (if they didn't).
In that context I'm not convinced it's grammatically incorrect, but apologies if the common HU parlance is confusing.
Grammar aside, I hope you'll be able to make the event. Looking forward to seeing your "pre-registration" come through.
By the way, Donington doesn't have a double "n".