Apex domains cannot have CNAME records.
Question and response
Question
dig awsdns-42.co.uk. NS
Answer
awsdns-42.co.uk. 5 NS g-ns-1838.awsdns-42.co.uk.
awsdns-42.co.uk. 5 NS g-ns-1517.awsdns-42.co.uk.
awsdns-42.co.uk. 5 NS g-ns-362.awsdns-42.co.uk.
awsdns-42.co.uk. 5 NS g-ns-938.awsdns-42.co.uk.
Authority
The response has no authority records.
Additional
g-ns-362.awsdns-42.co.uk. 5 A 205.251.193.106
g-ns-938.awsdns-42.co.uk. 5 A 205.251.195.170
g-ns-1517.awsdns-42.co.uk. 5 A 205.251.197.237
g-ns-1838.awsdns-42.co.uk. 5 A 205.251.199.46
g-ns-362.awsdns-42.co.uk. 5 AAAA 2600:9000:5301:6a00:0:0:0:1
g-ns-938.awsdns-42.co.uk. 5 AAAA 2600:9000:5303:aa00:0:0:0:1
g-ns-1517.awsdns-42.co.uk. 5 AAAA 2600:9000:5305:ed00:0:0:0:1
g-ns-1838.awsdns-42.co.uk. 5 AAAA 2600:9000:5307:2e00:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0