Apex domains cannot have CNAME records.
Question and response
Question
dig awsdns-31.co.uk. NS
Answer
awsdns-31.co.uk. 5 NS g-ns-1827.awsdns-31.co.uk.
awsdns-31.co.uk. 5 NS g-ns-1506.awsdns-31.co.uk.
awsdns-31.co.uk. 5 NS g-ns-351.awsdns-31.co.uk.
awsdns-31.co.uk. 5 NS g-ns-927.awsdns-31.co.uk.
Authority
The response has no authority records.
Additional
g-ns-351.awsdns-31.co.uk. 5 A 205.251.193.95
g-ns-927.awsdns-31.co.uk. 5 A 205.251.195.159
g-ns-1506.awsdns-31.co.uk. 5 A 205.251.197.226
g-ns-1827.awsdns-31.co.uk. 5 A 205.251.199.35
g-ns-351.awsdns-31.co.uk. 5 AAAA 2600:9000:5301:5f00:0:0:0:1
g-ns-927.awsdns-31.co.uk. 5 AAAA 2600:9000:5303:9f00:0:0:0:1
g-ns-1506.awsdns-31.co.uk. 5 AAAA 2600:9000:5305:e200:0:0:0:1
g-ns-1827.awsdns-31.co.uk. 5 AAAA 2600:9000:5307:2300:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0