Safari will not open sites

I was using Safari but it now seems that if your IP is supplied by BT then Safari is completely blocked from opening sites, other than BT ones, stating that the DNS address used is not compatible with BT.

This is a part of the message received

(You are seeing this page because you are using a BT service which requires the use of BT DNS servers and it appears that you are attempting to connect using a DNS server outside of our network.)

If I switch my browser over to Chrome then everything opens as normal

My system is OSX El Capitan, (which is causing the problem because it never happened till I downloaded this)

Version 10.11.6