I dislike the latest version of BT Yahoo Mail because of the idiotic adverts heading each mailbox. I only switched to it because I clicked in error but could find o way back. Calls to BT technical brought the response that it was impossible to switch back - but it is possible!
To the right of your mailbox, click on the Settings symbol (like a gear wheel), at the foot of the page there are 'More Settings', click and select the 'Switch back to BT Mail' in the left column and, voila, all is restored.
PS: I generally use Thunderbird to access my mailboxes and I have an Adblocker installed - a nice piece of software.