Apex domains cannot have CNAME records.
Question and response
Question
dig awsdns-16.co.uk. NS
Answer
awsdns-16.co.uk. 5 NS g-ns-1491.awsdns-16.co.uk.
awsdns-16.co.uk. 5 NS g-ns-1812.awsdns-16.co.uk.
awsdns-16.co.uk. 5 NS g-ns-336.awsdns-16.co.uk.
awsdns-16.co.uk. 5 NS g-ns-912.awsdns-16.co.uk.
Authority
The response has no authority records.
Additional
g-ns-336.awsdns-16.co.uk. 5 A 205.251.193.80
g-ns-912.awsdns-16.co.uk. 5 A 205.251.195.144
g-ns-1491.awsdns-16.co.uk. 5 A 205.251.197.211
g-ns-1812.awsdns-16.co.uk. 5 A 205.251.199.20
g-ns-336.awsdns-16.co.uk. 5 AAAA 2600:9000:5301:5000:0:0:0:1
g-ns-912.awsdns-16.co.uk. 5 AAAA 2600:9000:5303:9000:0:0:0:1
g-ns-1491.awsdns-16.co.uk. 5 AAAA 2600:9000:5305:d300:0:0:0:1
g-ns-1812.awsdns-16.co.uk. 5 AAAA 2600:9000:5307:1400:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0