Apex domains cannot have CNAME records.
Question and response
Question
dig awsdns-15.co.uk. NS
Answer
awsdns-15.co.uk. 5 NS g-ns-1811.awsdns-15.co.uk.
awsdns-15.co.uk. 5 NS g-ns-1490.awsdns-15.co.uk.
awsdns-15.co.uk. 5 NS g-ns-335.awsdns-15.co.uk.
awsdns-15.co.uk. 5 NS g-ns-911.awsdns-15.co.uk.
Authority
The response has no authority records.
Additional
g-ns-335.awsdns-15.co.uk. 5 A 205.251.193.79
g-ns-911.awsdns-15.co.uk. 5 A 205.251.195.143
g-ns-1490.awsdns-15.co.uk. 5 A 205.251.197.210
g-ns-1811.awsdns-15.co.uk. 5 A 205.251.199.19
g-ns-335.awsdns-15.co.uk. 5 AAAA 2600:9000:5301:4f00:0:0:0:1
g-ns-911.awsdns-15.co.uk. 5 AAAA 2600:9000:5303:8f00:0:0:0:1
g-ns-1490.awsdns-15.co.uk. 5 AAAA 2600:9000:5305:d200:0:0:0:1
g-ns-1811.awsdns-15.co.uk. 5 AAAA 2600:9000:5307:1300:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0