UCAS's translation definition covers its document upload service. Each university still sets its own entry rules. UCAS itself tells applicants that many course providers accept international qualifications equivalent to UK requirements, and to check the course university's website or contact them. You can also get a statement of how your qualifications compare to UK equivalents from UK ENIC.
There is a verification trap. UCAS states it is your responsibility to upload relevant and accurate documents, and that UCAS, schools, colleges and centres will not be able to check or verify them. Verification can come later from the university itself: Cambridge, for example, tells postgraduate offer holders they will be asked to send original documents so it can verify their authenticity. Keep your originals ready.
No UCAS page we checked mentions apostilles or legalisation of foreign qualification documents. That is a recorded silence, not a rule either way. If a university asks you to legalise a document, that is the university's own requirement, so confirm the details with them directly.