Apex domains cannot have CNAME records.
Question and response
Question
dig level3.co.uk. NS
Answer
level3.co.uk. 5 NS ns2.eu.level3.net.
level3.co.uk. 5 NS ns1.eu.level3.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0