Apex domains cannot have CNAME records.
Question and response
Question
dig aa.net.uk. NS
Answer
aa.net.uk. 2 NS secondary-dns.co.uk.
aa.net.uk. 2 NS primary-dns.co.uk.
Authority
The response has no authority records.
Additional
primary-dns.co.uk. 2 A 194.4.174.1
secondary-dns.co.uk. 2 A 194.4.173.1
primary-dns.co.uk. 2 AAAA 2001:8b0:0:30:0:0:51bb:1e29
secondary-dns.co.uk. 2 AAAA 2001:8b0:0:81:0:0:51bb:5120
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0