AHHH, okay, yes, this is a photocopy made by Kansas, no worries, that's what it's supposed to be, my mistake for having a brain hiccup on that.

So now I see what you mean about having to write the date in at this point.
The result form has already been processed/sent back, complete with sticker and...missing serum draw date, got it now!
If this were me, I would do exactly what you are doing -- ask Defra about this. But again I would actually give them a phone call; I called them during my cat's process and it really is the easiest and fastest way to hear it "from the horse's mouth" and put your mind at rest. After the usual automated menus you do actually get to talk to a real live person and the people on the phones truly know the answers.
At the worst, I would speculate that the USDA office who make the final sign off, and also the animal reception staff in the UK,
might take the date date that IS written in as the date to go by as a last resort.
But that's only my guess; please do get Defra to tell you the true story on this, as you never know what's going to matter in something like this. On the one hand there could already be plenty enough information for everyone to be satisfied and clearly see what's what, on the other it may be a wrinkle that has to be ironed out.
I would strongly suggest giving them a call rather than waiting on an e-mail reply; it will set your mind at rest much sooner so that whatever the answer is you can act upon it. I'm hoping that because there is at least a date on there and you do indeed have receipts to confirm the blood draw date, that this might not be an issue, but again, Defra are the ones who have the last word on that -- best wishes and give them a call. I'll see if I can find the number on the website and post back here.