Apex domains cannot have CNAME records.
Question and response
Question
dig net.uk. NS
Answer
net.uk. 5 NS nsb.nic.uk.
net.uk. 5 NS dns4.nic.uk.
net.uk. 5 NS dns2.nic.uk.
net.uk. 5 NS nsa.nic.uk.
net.uk. 5 NS nsc.nic.uk.
net.uk. 5 NS dns1.nic.uk.
net.uk. 5 NS nsd.nic.uk.
net.uk. 5 NS dns3.nic.uk.
Authority
The response has no authority records.
Additional
nsa.nic.uk. 67868 A 156.154.100.3
nsb.nic.uk. 67868 A 156.154.101.3
nsc.nic.uk. 67868 A 156.154.102.3
nsd.nic.uk. 67868 A 156.154.103.3
dns1.nic.uk. 67868 A 213.248.216.1
dns2.nic.uk. 67868 A 103.49.80.1
dns3.nic.uk. 67868 A 213.248.220.1
dns4.nic.uk. 67868 A 43.230.48.1
nsa.nic.uk. 67868 AAAA 2001:502:ad09:0:0:0:0:3
nsb.nic.uk. 67868 AAAA 2001:502:2eda:0:0:0:0:3
nsc.nic.uk. 67868 AAAA 2610:a1:1009:0:0:0:0:3
nsd.nic.uk. 67868 AAAA 2610:a1:1010:0:0:0:0:3
dns1.nic.uk. 67868 AAAA 2a01:618:400:0:0:0:0:1
dns2.nic.uk. 67868 AAAA 2401:fd80:400:0:0:0:0:1
dns3.nic.uk. 67868 AAAA 2a01:618:404:0:0:0:0:1
dns4.nic.uk. 67868 AAAA 2401:fd80:404:0:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0