I've been offered a trip to Scotland out of the blue tomorrow and the only film I have in the fridge is Delta 400, which I haven't tested yet.

I've only shot Delta 100 in the past, which I rate at ISO 50. I know Ilford are very consistent, but would it be foolish to rate Delta 400 at 200 with this in mind?
Should I go one step further and rate it 100? Does anyone else shoot both films, and what speed do you rate each? I'd like to get some relatively well exposed images on this trip.