I am using 1account for UK visitors who access our demo games, you can see how it works here https://onlinecasinoreviewz.com/online-slots/ if you click on one of the slot reviews, most have demo games. Anyone offering demo games to the UK market have to have a proper age verification system in place. The integration side of it can be a bit tricky, I had my developer help me with that, but it works great. Not sure how you would integrate it into a email subscriber form, I should imagine the more obstacles you put in the way, then the less likely people are to subscribe.
"Many of life's failures are people who did not realize how close they were to success when they gave up"