I could be wrong, but in my
opinion with all that roasted barley in there, the American 2-row replacing the British is not going be so big an issue...especially if you're aiming at a dry stout.
Throw some Munich or something in there if it makes you feel better, but if it were me, I'd just use what you have. It should be fine. I'd say just go for it.
Others will weigh in, I'm sure. Add up the responses, and take an average.