Apex domains cannot have CNAME records.
Question and response
Question
dig unitedvacations.co.uk. NS
Answer
unitedvacations.co.uk. 5 NS udc-gtm01.uls-prod.com.
unitedvacations.co.uk. 5 NS ndc-gtm01.uls-prod.com.
unitedvacations.co.uk. 5 NS ndc-gtm02.uls-prod.com.
unitedvacations.co.uk. 5 NS udc-gtm02.uls-prod.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0