Apex domains cannot have CNAME records.
Question and response
Question
dig awsdns-25.co.uk. NS
Answer
awsdns-25.co.uk. 5 NS g-ns-345.awsdns-25.co.uk.
awsdns-25.co.uk. 5 NS g-ns-1821.awsdns-25.co.uk.
awsdns-25.co.uk. 5 NS g-ns-1500.awsdns-25.co.uk.
awsdns-25.co.uk. 5 NS g-ns-921.awsdns-25.co.uk.
Authority
The response has no authority records.
Additional
g-ns-345.awsdns-25.co.uk. 5 A 205.251.193.89
g-ns-921.awsdns-25.co.uk. 5 A 205.251.195.153
g-ns-1500.awsdns-25.co.uk. 5 A 205.251.197.220
g-ns-1821.awsdns-25.co.uk. 5 A 205.251.199.29
g-ns-345.awsdns-25.co.uk. 5 AAAA 2600:9000:5301:5900:0:0:0:1
g-ns-921.awsdns-25.co.uk. 5 AAAA 2600:9000:5303:9900:0:0:0:1
g-ns-1500.awsdns-25.co.uk. 5 AAAA 2600:9000:5305:dc00:0:0:0:1
g-ns-1821.awsdns-25.co.uk. 5 AAAA 2600:9000:5307:1d00:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0