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You are only permitted to terminate the tenancy early if there is a break clause entitling you to do so or if the property is unfit for human habitation. You would have to prove unfitness by using evidence to the Court.
Otherwise you are bound by the full term of the rent to the landlord and he will be able to sue you for it. He will likely obtain judgement at Court for rent that you would have paid up to the point at which the Court decides he would have been able to re-let the property.
However, landlords are generally under a duty not to unreasonably refuse a suitable tenant and your focus now should be on finding one. If the landlord acted through agents then you should explain your situation to them and press them (and continue to press them) to find you a replacement tenant. If the landlord acted on his own then you should speak to him to check that he would be amenable to your finding a replacement tenant. You can then use http://www.gumtree.co.uk/ to find a tenant (private landlords frequently use this service.
If you were occupying the property under an assured shorthold tenancy agreement ) then your landlord must put the deposit monies in to a tenancy deposit scheme within 14 days of receipt. All deposit under ASTs have to be placed in such a scheme and you can sue the landlord for the return of the deposit and a fine of three times the amout of the deposit if it has not been placed in a TDS. Ask for details of the scheme in to which the deposit has been placed.If it hasn't then you can either use it as leverage to get out of the dispute (suggesting you will take her to Court because of the deposit not being properly deposited), or you can issue a claim against her at Court for the money.
I am afraid though that you are bound by the tenancy agreement regardless of the change in yoru personal circumstances. If it sounds as if he is going to issue a claim against you for the money then you should plead poverty to him and state that you have no assets or income against which judegment could be enforced. This may or may not stop him from issuing.