Apex domains cannot have CNAME records.
Question and response
Question
dig gslb21.ic.ac.uk. NS
Answer
gslb21.ic.ac.uk. 4 NS gslb3.net.ic.ac.uk.
gslb21.ic.ac.uk. 4 NS gslb4.net.ic.ac.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0