On a mobile device towards the top right of the page, you should have "mobile menu" icon (as shown in the following image). Tap this.
The mobile menu will open. The top menu item (above "Membership: Join Online") is "Existing user? Sign In".
Tap the "Existing user? Sign In" item and then fill in your username and password. (Note: Your username and password for the BN main website will be the same as your username and password for the BN membership system - that is, if you have an account on the BN membership system.)
If you have forgotten your username or password then please refer to this guide: https://www.bn.org.uk/guides/not-logged-in/what-is-my-username-and-password-members-only-r10/
If you find any errors or want to provide other feedback about this guide then please send email to firstname.lastname@example.org. Thank-you!