Ending contract early

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I am jointly liable under a contract for a shared flat with 2 other people. Before Corona, I informed my landlady I would be leaving for a placement in Bradford and I had found a replacement. She accepted this However, when Corona worsened, the replacement returned to her home country and my dad brought me to my hometown to avoid being trapped in London in the impending lockdown. I cleared my room before leaving (although I have some things left in the attic). I asked for a rent holiday or reduction but my landlady refused. She said I must find a replacement in order to leave early, and there is no break out clause in the contract. I found a replacement but she said the others have to agree. One of the girls insisted on another female living in the flat so she refused, and then my landlady also refused on that basis. There is no specific requirement in the contract for me to find a replacement but it says I can only surrender with the landlady's written consent and 'subject to certain conditions that may included the landlord's reasonable costs associated with the re-letting of the premises'. If I refuse to pay, my share of the rent falls on the other tenants.
Do I have any rights as a tenant to end the contract early? Is a contract still valid without a breakout clause?
Thank you.