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Thank you for your question
My name is ***** ***** I shall do my best to help you
I am afraid that the agency is correct.
You took the tenancy on a "joint and several basis" and you can only leave and no longer be responsible for the rent if either
you BOTH move out or
your landlord agrees to release you from the tenancy and issue a new tenancy to your former partner alone.
Of course you are free to move out BUT you remain liable for the rent if the other person fails to pay.
I am sorry that the news is so bad - please ask if you need further details