Shouldn't that get caught by the Public Suffix List?
I would hope DigiCert has checks in place to prevent someone domain-validating ownership of the entire of co.uk under any circumstances :)
(They should still revoke the mis-issued certificates though)
BR 3.2.2.6 prohibits issuing a wildcard certificate for an entire public suffix unless the "Applicant proves its rightful control of the entire Domain Namespace" (without specifying how this should be done - arguably, publishing a DNS record would qualify) but also says that CAs should use the "ICANN DOMAINS" section of the PSL only, not the "PRIVATE DOMAINS" section, so domains for dynamic DNS providers and the like wouldn't be included. [https://github.com/cabforum/servercert/blob/main/docs/BR.md#...]
PSL is a best-effort sort of thing, so it's good but not definitive. It would be dangerous to rely on it when issuing certs imo.