Apex domains cannot have CNAME records.
Question and response
Question
dig awsdns-21.co.uk. NS
Answer
awsdns-21.co.uk. 5 NS g-ns-341.awsdns-21.co.uk.
awsdns-21.co.uk. 5 NS g-ns-1496.awsdns-21.co.uk.
awsdns-21.co.uk. 5 NS g-ns-917.awsdns-21.co.uk.
awsdns-21.co.uk. 5 NS g-ns-1817.awsdns-21.co.uk.
Authority
The response has no authority records.
Additional
g-ns-341.awsdns-21.co.uk. 5 A 205.251.193.85
g-ns-917.awsdns-21.co.uk. 5 A 205.251.195.149
g-ns-1496.awsdns-21.co.uk. 5 A 205.251.197.216
g-ns-1817.awsdns-21.co.uk. 5 A 205.251.199.25
g-ns-341.awsdns-21.co.uk. 5 AAAA 2600:9000:5301:5500:0:0:0:1
g-ns-917.awsdns-21.co.uk. 5 AAAA 2600:9000:5303:9500:0:0:0:1
g-ns-1496.awsdns-21.co.uk. 5 AAAA 2600:9000:5305:d800:0:0:0:1
g-ns-1817.awsdns-21.co.uk. 5 AAAA 2600:9000:5307:1900:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0