Apex domains cannot have CNAME records.
Question and response
Question
dig dhl.co.uk. NS
Answer
dhl.co.uk. 4 NS ns2b.dhl.com.
dhl.co.uk. 4 NS ns2.dhl.com.
dhl.co.uk. 4 NS ns6b.dhl.com.
dhl.co.uk. 4 NS ns4.dhl.com.
dhl.co.uk. 4 NS ns6.dhl.com.
dhl.co.uk. 4 NS ns4b.dhl.com.
Authority
The response has no authority records.
Additional
ns2.dhl.com. 4 A 156.137.1.166
ns4.dhl.com. 4 A 165.72.192.16
ns6.dhl.com. 4 A 199.40.254.166
ns2b.dhl.com. 4 A 156.137.195.66
ns4b.dhl.com. 4 A 165.72.190.166
ns6b.dhl.com. 4 A 199.40.15.66
ns2.dhl.com. 4 AAAA 2620:1d6:fff3:410a:0:0:0:1
ns4.dhl.com. 4 AAAA 2a05:4a40:fff3:410a:0:0:0:1
ns6.dhl.com. 4 AAAA 2403:c580:fff3:410a:0:0:0:1
ns2b.dhl.com. 4 AAAA 2620:1d6:fffd:410a:0:0:0:1
ns4b.dhl.com. 4 AAAA 2a05:4a40:fffd:410a:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0