Apex domains cannot have CNAME records.
Question and response
Question
dig awsdns-28.co.uk. NS
Answer
awsdns-28.co.uk. 5 NS g-ns-348.awsdns-28.co.uk.
awsdns-28.co.uk. 5 NS g-ns-924.awsdns-28.co.uk.
awsdns-28.co.uk. 5 NS g-ns-1503.awsdns-28.co.uk.
awsdns-28.co.uk. 5 NS g-ns-1824.awsdns-28.co.uk.
Authority
The response has no authority records.
Additional
g-ns-348.awsdns-28.co.uk. 5 A 205.251.193.92
g-ns-924.awsdns-28.co.uk. 5 A 205.251.195.156
g-ns-1503.awsdns-28.co.uk. 5 A 205.251.197.223
g-ns-1824.awsdns-28.co.uk. 5 A 205.251.199.32
g-ns-348.awsdns-28.co.uk. 5 AAAA 2600:9000:5301:5c00:0:0:0:1
g-ns-924.awsdns-28.co.uk. 5 AAAA 2600:9000:5303:9c00:0:0:0:1
g-ns-1503.awsdns-28.co.uk. 5 AAAA 2600:9000:5305:df00:0:0:0:1
g-ns-1824.awsdns-28.co.uk. 5 AAAA 2600:9000:5307:2000:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0