Apex domains cannot have CNAME records.
Question and response
Question
dig awsdns-23.co.uk. NS
Answer
awsdns-23.co.uk. 5 NS g-ns-343.awsdns-23.co.uk.
awsdns-23.co.uk. 5 NS g-ns-1498.awsdns-23.co.uk.
awsdns-23.co.uk. 5 NS g-ns-919.awsdns-23.co.uk.
awsdns-23.co.uk. 5 NS g-ns-1819.awsdns-23.co.uk.
Authority
The response has no authority records.
Additional
g-ns-343.awsdns-23.co.uk. 5 A 205.251.193.87
g-ns-919.awsdns-23.co.uk. 5 A 205.251.195.151
g-ns-1498.awsdns-23.co.uk. 5 A 205.251.197.218
g-ns-1819.awsdns-23.co.uk. 5 A 205.251.199.27
g-ns-343.awsdns-23.co.uk. 5 AAAA 2600:9000:5301:5700:0:0:0:1
g-ns-919.awsdns-23.co.uk. 5 AAAA 2600:9000:5303:9700:0:0:0:1
g-ns-1498.awsdns-23.co.uk. 5 AAAA 2600:9000:5305:da00:0:0:0:1
g-ns-1819.awsdns-23.co.uk. 5 AAAA 2600:9000:5307:1b00:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0