Apex domains cannot have CNAME records.
Question and response
Question
dig awsdns-19.co.uk. NS
Answer
awsdns-19.co.uk. 5 NS g-ns-915.awsdns-19.co.uk.
awsdns-19.co.uk. 5 NS g-ns-1815.awsdns-19.co.uk.
awsdns-19.co.uk. 5 NS g-ns-339.awsdns-19.co.uk.
awsdns-19.co.uk. 5 NS g-ns-1494.awsdns-19.co.uk.
Authority
The response has no authority records.
Additional
g-ns-339.awsdns-19.co.uk. 5 A 205.251.193.83
g-ns-915.awsdns-19.co.uk. 5 A 205.251.195.147
g-ns-1494.awsdns-19.co.uk. 5 A 205.251.197.214
g-ns-1815.awsdns-19.co.uk. 5 A 205.251.199.23
g-ns-339.awsdns-19.co.uk. 5 AAAA 2600:9000:5301:5300:0:0:0:1
g-ns-915.awsdns-19.co.uk. 5 AAAA 2600:9000:5303:9300:0:0:0:1
g-ns-1494.awsdns-19.co.uk. 5 AAAA 2600:9000:5305:d600:0:0:0:1
g-ns-1815.awsdns-19.co.uk. 5 AAAA 2600:9000:5307:1700:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0