This is not terribly uncommon (although not widely talked about). It'll almost certainly be a Google Global Cache setup (https://peering.google.com/about/ggc.ht

This is not terribly uncommon (although not widely talked about). It'll almost certainly be a Google Global Cache setup (https://peering.google.com/about/ggc.html https://peering.google.com/about/ggc.html). You might find it's Google, not BT, that is sending you to a different set of servers depending on your source address (EDIT: likely using EDNS, so you could test this from a non-BT host using a specially crafted DNS query).