Apex domains cannot have CNAME records.
Question and response
Question
dig gia.bt.net. NS
Answer
gia.bt.net. 5 NS ns1eu.bt.net.
gia.bt.net. 5 NS ns2eu.bt.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0