Yeah, I think if I'd attached the form (maybe with a highlight on the blank field), it would've been a bit clearer, and even on the phone I don't think I really accurately described what I meant, but she was able to get the idea.

I filled in the field myself, after all that, because I figured the chances of hte vet actually doing it (and putting the right date

) are slim to none, and there's no way DEFRA will know/give a crap anyway. Just as a CYA thing, though, I'm going to draft a letter for my vet's office saying that we did follow all the procedures correctly (as far as microchip first, then rabies, then blood work, etc), and have them print it on letterhead and the vet sign it. Not sure if it'll help, but it can't hurt.
(So now my only nagging fear is that they'll see 6/12, which is the blood draw date, and the vet's signature from 6/15, and they'll think he didn't verify microchip # before they drew the blood, but I did specifically ask them to do that.)