From a technical aspect, the domain name ox.ac.uk is just that, a domain name. It is not specifying any particular service such as Web, Email, or FTP. While in theory a hostname matching the domain name may exist, there is a problem as to what service(s) should be associated with that hostname. Nowadays one would not choose to run multiple services from one host, for reasons of resilience and security amongst others. Therefore when the hostname ox.ac.uk exists there is an implicit acceptance that this is for the use of one service at the exclusion of any other.
The problem was solved for Email many years ago with the addition of a dedicated DNS record type (the MX record), allowing for complex setups for a mail domain with a name matching that of the domain. A more general method does exist for other services in the form of the SRV record, but outside of certain areas (such as MIT Kerberos systems or Windows domains) they are not widely used. Standard practice has instead been to use service-specific hostnames, such as www.ox.ac.uk, ftp.ox.ac.uk, smtp.ox.ac.uk, although in principle, any name can generally be used for the server offering any service.
A handful of legacy DNS records of the form unit.ox.ac.uk do exist, although we have never had an A record for ox.ac.uk itself. Mostly these were originally set up for use with departmental "bastion" hosts, often used as telnet (and later ssh) gateways. These days we would recommend that a name such as login.unit.ox.ac.uk be associated with such systems.