The Foreign and Commonwealth Office has re-opened its Legalisation Office, although with limited capacity at present, so apostilles are taking longer than usual to process. The current position may be seen at: https://www.get-document-legalised.service.gov.uk/select-service. This link also includes a letter from the FCO that can be downloaded describing an emergency procedure.
Documents will need to be submitted by a consular agent registered with the FCO. My consular agent is one of the agents permitted to do this.
Some embassies are currently providing legalisation services, and some have closed. If you have a document that needs urgent legalisation, please let me know and enquiries can be made as to the status of the relevant embassy.