Update: coincidentally, I just got a formal cancellation email from Eurostar today. It tried to steer me in the direction of a voucher or reschedule, but after about 12 clicks I did find an option for refund online. Submitted and fingers crossed.

Update 2: refund posted to credit card about a week later with no issues. Pleasantly surprised after all the horror stories I heard about Eurostar. I even made $10 on the net transactions due to changes in exchange rates.