How much storage does the BT Cloud website say you're using (to check, go to https://btcloud.bt.com, then click your name at the top right of the screen, then 'My Settings')?
If it's also showing the 50GB is being used then the data is still on your BT Cloud account. You can check the Deleted Items folder (this also counts towards your usage limit), and also you can look in All My Files to see anything that's still backed up. You can find instructions on how to use these features here.
If the BT Cloud website is now showing this space is free then it sounds like the BT Cloud desktop app hasn't yet refreshed itself to check your new quota. It does this periodically, but you can trigger it yourself by closing and then reopening the app.