Apex domains cannot have CNAME records.
Question and response
Question
dig awsdns-51.co.uk. NS
Answer
awsdns-51.co.uk. 5 NS g-ns-1847.awsdns-51.co.uk.
awsdns-51.co.uk. 5 NS g-ns-947.awsdns-51.co.uk.
awsdns-51.co.uk. 5 NS g-ns-371.awsdns-51.co.uk.
awsdns-51.co.uk. 5 NS g-ns-1526.awsdns-51.co.uk.
Authority
The response has no authority records.
Additional
g-ns-371.awsdns-51.co.uk. 5 A 205.251.193.115
g-ns-947.awsdns-51.co.uk. 5 A 205.251.195.179
g-ns-1526.awsdns-51.co.uk. 5 A 205.251.197.246
g-ns-1847.awsdns-51.co.uk. 5 A 205.251.199.55
g-ns-371.awsdns-51.co.uk. 5 AAAA 2600:9000:5301:7300:0:0:0:1
g-ns-947.awsdns-51.co.uk. 5 AAAA 2600:9000:5303:b300:0:0:0:1
g-ns-1526.awsdns-51.co.uk. 5 AAAA 2600:9000:5305:f600:0:0:0:1
g-ns-1847.awsdns-51.co.uk. 5 AAAA 2600:9000:5307:3700:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0