For international students unable - owing to the rules - to come to the UK for study this coming semester, there should be an alternative option offered by your university.
Your best option is to contact them directly - it will likely be Registry who are dealing with this (likely also on a case-by-case basis) so you can contact them directly, if there's no Covid microsite, or you've not been told a different point of contact. Alternatively, ask your personal tutor/head of year/head of department/departmental administrator what you need to do - or who, if they don't know, it is you need to ask.