Apex domains cannot have CNAME records.
Question and response
Question
dig awsdns-56.co.uk. NS
Answer
awsdns-56.co.uk. 5 NS g-ns-1852.awsdns-56.co.uk.
awsdns-56.co.uk. 5 NS g-ns-1531.awsdns-56.co.uk.
awsdns-56.co.uk. 5 NS g-ns-376.awsdns-56.co.uk.
awsdns-56.co.uk. 5 NS g-ns-952.awsdns-56.co.uk.
Authority
The response has no authority records.
Additional
g-ns-376.awsdns-56.co.uk. 5 A 205.251.193.120
g-ns-952.awsdns-56.co.uk. 5 A 205.251.195.184
g-ns-1531.awsdns-56.co.uk. 5 A 205.251.197.251
g-ns-1852.awsdns-56.co.uk. 5 A 205.251.199.60
g-ns-376.awsdns-56.co.uk. 5 AAAA 2600:9000:5301:7800:0:0:0:1
g-ns-952.awsdns-56.co.uk. 5 AAAA 2600:9000:5303:b800:0:0:0:1
g-ns-1531.awsdns-56.co.uk. 5 AAAA 2600:9000:5305:fb00:0:0:0:1
g-ns-1852.awsdns-56.co.uk. 5 AAAA 2600:9000:5307:3c00:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0