I've contacted the billing team. I had a good conversation with a representative. He suggested I do a factory reset, which was something of an effort, but I did it, and no success.
Then the Roku rep said
"Thank you for contacting Roku customer support. We understand that you didn't get the BritBox 30 day free trial when you activated the Roku device.
We like to inform you that the issue seems to be with the specific channel. We suggest you contact the BritBox channel provider to report the issue and get further assistance in activating the free trial."
So I contacted Britbox, who replied
"Thanks for contacting BritBox.
So sorry to hear that you're experiencing an issue with your account. I can understand how frustrating this can be.
I've tried to locate your account on our system using the email address provided, but no results!
Do you have any other email address(es) that you could of used to create an account, if so please provide?
If you have signed up to Britbox through the third party vendor Roku, then the subscription is with them. We only provide the content to their customers. It's Roku who activates the entitlement on the subscription.
Any issues with the account would need to be taken up with Roku, not us!"
So each side is blaming the other, and I am not getting the offer you advertised. I expect you will be able to settle the matter somehow, but for the moment I would warn people about the offer, and might even contact the Advertising Standards Authority over false advertising.