Your thought process in choosing 1250 was as good as it gets.

Delta 3200 is an ISO 1000 film so you actually shot at almost box speed but that doesn't matter that much, you can get reasonable results shot anywhere from 400-12500.

The closest times Ilford publishes are for 800 & 1600. 1600 is the closest so I'd instruct the lab to "develop for EI 1600".