Apex domains cannot have CNAME records.
Question and response
Question
dig awsdns-40.co.uk. NS
Answer
awsdns-40.co.uk. 5 NS g-ns-936.awsdns-40.co.uk.
awsdns-40.co.uk. 5 NS g-ns-360.awsdns-40.co.uk.
awsdns-40.co.uk. 5 NS g-ns-1836.awsdns-40.co.uk.
awsdns-40.co.uk. 5 NS g-ns-1515.awsdns-40.co.uk.
Authority
The response has no authority records.
Additional
g-ns-360.awsdns-40.co.uk. 5 A 205.251.193.104
g-ns-936.awsdns-40.co.uk. 5 A 205.251.195.168
g-ns-1515.awsdns-40.co.uk. 5 A 205.251.197.235
g-ns-1836.awsdns-40.co.uk. 5 A 205.251.199.44
g-ns-360.awsdns-40.co.uk. 5 AAAA 2600:9000:5301:6800:0:0:0:1
g-ns-936.awsdns-40.co.uk. 5 AAAA 2600:9000:5303:a800:0:0:0:1
g-ns-1515.awsdns-40.co.uk. 5 AAAA 2600:9000:5305:eb00:0:0:0:1
g-ns-1836.awsdns-40.co.uk. 5 AAAA 2600:9000:5307:2c00:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0