Apex domains cannot have CNAME records.
Question and response
Question
dig onetel.net.uk. NS
Answer
onetel.net.uk. 1 NS ds1.opaltelecom.net.
onetel.net.uk. 1 NS ds0.opaltelecom.net.
onetel.net.uk. 1 NS ds2.opaltelecom.net.
Authority
The response has no authority records.
Additional
ds0.opaltelecom.net. 1 A 81.1.70.10
ds1.opaltelecom.net. 1 A 185.175.50.10
ds2.opaltelecom.net. 1 A 212.1.130.10
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0