Just had a frustrating hour on the phone to support.
The issue was that my wife was getting a BT page saying access had been blocked by Parental Controls due to the use of non-BT DNS servers. This was when accessing her work portal using WiFi on a Macbook Pro running Firefox. It had previously worked OK and Parental Controls had never been turned on on our broadband connection.
After much fruitless turning on and off of Parental controls and rebooting at the behest of the hapless customer service rep who is at the mercy of the faceless back office staff who never speak to customers here's the answer.
We don't use a Home Hub, we use a Cisco Router because we also use a VPN. That router had some non-BT DNS servers set up so when a user connected they would be given those DNS servers by DHCP. Never a problem because Parental Controls are off.
She then visited her mother, who has BT Broadband with Parental Controls turned on. When she tried to do some work by logging on to her employer's portal she got redirected to the BT blocking page for not using BT DNS servers. For Parental Controls to work you have to be using BT DNS Servers so if you are using someone else's you will get blocked if you use any BT Broadband that has Parental Controls turned on.
This redirect than got stored in the Mozilla cache on her Macbook so from then on whenever she wanted to go to her employer's portal she got blocked, no matter where she was. Back home she wast blocked by Mozilla but she could connect using Safari because of course she had never had access blocked in Safari so the redirect was not stored in Safari's cache.
Clearing Mozilla's cache cured the problem and she could reconnect.
This is behaviour that BT should have anticipated and for that reason they should not have introduced it. If you once connect to WiFi with BT Parental Controls turned on and get blocked, that redirect is stored ion your cache and will continue to operate even after Parental Controls are turned off and even if you switch to a non-BT WiFi with no parental controls. Effectively it is a breach of the Computer Misuse Act because BT is interfereing with data held on your computer. They have the expertise to know that this would be a result of their actions. BT's information on how to clear the problem that you are directed to by the blocking page is inaccurate and insufficient to guide anyone to a resolution. It does not include any advice to clear the cache.
It doesn't matter a jot what address you are trying to get to. In this case it was myscad.scad.edu the web portal for all students and faculty at Savannah College of Art and Design, the American university where she is a professor. You don't have to be doing something you shouldn't to have BT screw up your browser, all that's necessary is for you to use a BT circuit with Parental Controls turned on when you are not using BT's DNS servers. You might for example prefer to use Google's or Open DNS. The redirect to the BT Blocking page will be cached in your browser.
Don't expect the BT staff you get on the end of the line to be able to work this one out. I hope this helps you if you end up in the same situation.
Sounds like the "site blocked page" may not contain the right "do not cache" headers.
If you aren't using BT vision or Youview I recommend getting a different router, setting OpenDNS as your DNS service on it (works WAY WAY better than Parental controls imho) and blocking DNS port (only needed if there is anyone in your family techie enough to try to circumvent OpenDNS by overriding PC DHCP DNS settings).
BT Home hub is apparently the only router made where the DNS setting cannot be changed, so without changing it you are stuck with Parental controls or nothing ... unles you switch provider of course.
To use any non-BT DNS servers, BT Parental controls have to be un-installed, not just turned off. You then have to restart the home hub, and clear any browser caches.
It does not matter which router you use, as the controls are applied on the BT network. Restarting the router simply makes sure you get a different IP address, and clears any DNS cache.
Your router must use BTs DNS servers, otherwise all sites will be blocked.
I've just had the same problem with using Firefox Mozilla to one particular site after switching on BT Protect and then switching it back off. Clearing the cache as suggested resolved the problem, which was not the solution suggested in the BT instructions to overcome the problem.