Nothing that involves Cloudflare, as the information you have provided suggests that part is correct. Does the autodiscoverCNAME resolve correctly on the device that is running Outlook?
This is the result from the Test Email Auto Configuration:-
Autodiscover to` https://autodiscover.xxxx.co.uk/autodiscover/autodiscover.xml` starting
GetLastError=2147954429;httpStatus=0.
Autodiscover to `https://autodiscover.xxxx.co.uk/autodiscover/autodiscover.xml` Failed (ox80070057)
Local Autodiscover for xxxxx.co.uk starting
Local Autodiscover for xxxxx.co.uk Failed (0x8004010F)
Redirect check to `https://autodiscover.xxxxx.co.uk/autodiscover/autodiscover.xml` starting
Autodiscover URL redirection to `https://autodiscover-s.outlook.com/autodiscover/autodiscover.xml`
Autodiscover to `https://autodiscover-s.outlook.com/autodiscover/autodiscover.xml' starting
GetLastError=0;httpStatus=401.
GetLastErro =0;httpStatus=200.
Autodiscover to `https://autodiscover-s.outllok.com/autodiscover/autodiscover.xml` starting Failed (0x800C8203)
Redirect to `https://autodiscover-s.outllok.com/autodiscover/autodiscover.xml` starting Failed (0x800C8203)
Srv Record lookup for xxxxx.co.uk Starting
Srv Record lookup for xxxxx.co.uk Failed (0x8004010F)
That doesn’t look right. However, what you provided isn’t what I want to see. I just want the results of a command line DNS query for the autodiscoverCNAME performaed on one of your hosts that is having trouble with Outlook. My expectation is that you have something wrong in your local DNS environment and those results should confirm that for you.