Yes, reward codes now work on subscriptions too! You'll need to select one of the subscription specific discounts, these can then be applied to your next subscription order via the customer portal on your account :)
Log into your account at https://www.ozonecoffee.co.uk/account and click on the Manage Subscriptions link on the right-hand side of the page to go to your subscription management portal. From there you can enter your reward code by clicking the Add discount button.