Whilst I don't use a VPN, there are some sites that can tell if you are using one and block it. How they do this I don't know.
In the forums if my memory serves me right, I have seen something in the forums about using 'Wonderland' as a VPN selection to see if that can bypass this restriction. I'm not sure if this is also an option in the Avast SecureLine VPN for iOS.
I think the BBC is very proactive in trying to block VPN use, as to use it you have to be a BBC licence payer.