Apex domains cannot have CNAME records.
Question and response
Question
dig awsdns-20.co.uk. NS
Answer
awsdns-20.co.uk. 5 NS g-ns-916.awsdns-20.co.uk.
awsdns-20.co.uk. 5 NS g-ns-340.awsdns-20.co.uk.
awsdns-20.co.uk. 5 NS g-ns-1816.awsdns-20.co.uk.
awsdns-20.co.uk. 5 NS g-ns-1495.awsdns-20.co.uk.
Authority
The response has no authority records.
Additional
g-ns-340.awsdns-20.co.uk. 5 A 205.251.193.84
g-ns-916.awsdns-20.co.uk. 5 A 205.251.195.148
g-ns-1495.awsdns-20.co.uk. 5 A 205.251.197.215
g-ns-1816.awsdns-20.co.uk. 5 A 205.251.199.24
g-ns-340.awsdns-20.co.uk. 5 AAAA 2600:9000:5301:5400:0:0:0:1
g-ns-916.awsdns-20.co.uk. 5 AAAA 2600:9000:5303:9400:0:0:0:1
g-ns-1495.awsdns-20.co.uk. 5 AAAA 2600:9000:5305:d700:0:0:0:1
g-ns-1816.awsdns-20.co.uk. 5 AAAA 2600:9000:5307:1800:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0