I have enabled and blocked youtube since 3-4 days ago but still the website is available.
I followed the instructions here (https://www.bt.com/help/security/how-to-keep-your-family-safe-online-with-bt-parental-controls-an#bl...) and blocked all 4 URLs mentioned (btw the guide screenshots are quite old and don't match what I see in admin page, it also says it takes 2 hours for settings to be updated while the email I get says 10 minutes). But still Youtube can be visited.
Does this even work?