You need to post here further details of the terms on which you paid the "holding deposit", and the terms on which you paid the rent in advance.
Did you appoint the other tenant as your agent, to sign a tenancy agreement on your behalf? The court is likely to conclude that you did, if you paid a month's rent on the property. If so, you are bound by the tenancy agreement.
To terminate a tenancy, you must give due notice. If you have not done so, the tenancy still exists and you are liable for the rent until you do now validly terminate it.
You cannot terminate a tenancy for a fixed term, such as 6 months, unless the landlord agrees; so, if the tenancy is for a fixed term, you are reliant on the landlord's goodwill. Is he willing to agree a surrender of the tenancy, before the 6 months are up? It cannot be terminated in that way except with his agreement.
The only exception is if the tenancy agreement contains a clause allowing the tenant to give notice to terminate it early, in which case you must now give the proper notice as required by that document.
Alternatively, if you have a tenancy that is periodic, i.e. which does not have an initial fixed term, you must still give notice to terminate it: one rental period's notice (i.e. usually one clear calendar month's notice), in writing, expiring on the day before a rent day.
Such a tenancy can, alternatively, be surrendered early, if the landlord agrees.
Read this thread for some further information:
http://www.consumeractiongroup.co.uk...-eviction.html
I am assuming that you have a shorthold tenancy, as explained in that thread. Your legal position may be different if it is not a shorthold tenancy.
Advice & opinions on this forum are offered informally, without any assumption of liability. Use your own judgment. Seek advice of a qualified and insured professional if you have any doubts.