The best way to cancel your subscription is if you do it yourself – then the system knows it’s happened and it’s less likely that there will be errors.
First, go to the course site at https://courses.learning-to-see.co.uk/ and log in using the “sign in” link at the top right:

Once you’re signed in, click the little icon at the top right, then select “My Account” from the drop down menu:

That will take you to this page. Now click the “billing” link on the left:

On the next page, you’ll see your subscription with a link to cancel. Just click that link. You’ll still have access to all the materials until the end of the current billing cycle.
You should also still have access to any courses that you bought separately, just by logging in to the courses site in the same way.