Apex domains cannot have CNAME records.
Question and response
Question
dig awsdns-33.co.uk. NS
Answer
awsdns-33.co.uk. 5 NS g-ns-1508.awsdns-33.co.uk.
awsdns-33.co.uk. 5 NS g-ns-353.awsdns-33.co.uk.
awsdns-33.co.uk. 5 NS g-ns-929.awsdns-33.co.uk.
awsdns-33.co.uk. 5 NS g-ns-1829.awsdns-33.co.uk.
Authority
The response has no authority records.
Additional
g-ns-353.awsdns-33.co.uk. 5 A 205.251.193.97
g-ns-929.awsdns-33.co.uk. 5 A 205.251.195.161
g-ns-1508.awsdns-33.co.uk. 5 A 205.251.197.228
g-ns-1829.awsdns-33.co.uk. 5 A 205.251.199.37
g-ns-353.awsdns-33.co.uk. 5 AAAA 2600:9000:5301:6100:0:0:0:1
g-ns-929.awsdns-33.co.uk. 5 AAAA 2600:9000:5303:a100:0:0:0:1
g-ns-1508.awsdns-33.co.uk. 5 AAAA 2600:9000:5305:e400:0:0:0:1
g-ns-1829.awsdns-33.co.uk. 5 AAAA 2600:9000:5307:2500:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0