There is no straight forward answer to this - it depends really on your contract and your landlord!
Use the Shelter tenancy checker to verify your tenancy and this will confirm your rights - addy http://england.shelter.org.uk/get_ad...enancy_checker
Basically if you are an assured shorthold tenant (most are in private rented) then if you have signed a 1n 18 months contract you have agreed to that period of time ... unless ...
a) your contract contains a break clause - basically is a clear term that you can give a specific amount of notice at certain points to terminate your tenancy early
b) if no break clause - the only other option is to agree a mutual surrender with your landlord (that is where you and any other tenants and the landlord agree that on a specific date the tenancy will end early) this would discharge any further rent liability
If neither of the above, if you left you could be held responsible for your rent for the fixed term.
Hope the above makes sense .. any questions email or ring Shelter!