www.gov.uk/government/publications/visiting-care-homes-during-coronavirus/update-on-policies-for-visiting-arrangements-in-care-homes
Guidance
Update on policies for visiting arrangements in care homes
First of all the key thing here is there doesn't appear to be a law regarding visting arrangements in care homes.
This document is GUIDANCE (dated 21st September and the most recent one). It ADVISES no more than one visitor per person.
But crucially this is ADVICE. It is NOT law.
This means there is room on compassionate grounds to allow a single last visit if the care home is willing to accomodate it.
I believe the position is that several local councils have restricted visits to council owned establishments - but private homes may fall outside this.
Without knowing what council you are talking about, I can't look up what the local restrictions are - but again since they are NOT law - my suspicion is that there is an ability for end life visits if the care home is willing to facilitate it.
My gut is you are being told this is law to fob you off and stop you challenging them because they don't want to facilitiate an end of life visit.
The problem is I don't think you have a right to visit her either, so they can be difficult over this if they want to.
But I do think you are being given information which isn't strictly true and it might mean you may be able to challenge them if this is the case.