Hello, I am a solicitor with over 15 years experience. I will try to help you with this.
You have probably signed a tenancy agreement jointly with your prospective housemates. You are right in your second interpretation of the clause you refer to and wrong in your first interpretation.
does that mean that there is no other way of cancelling the contract other than finding a replacement?
well and obviously if the Landlord would agree to a surrender
The situation is that if you have signed up to pay rent you will need to do so unless you find a replacement. Unless there is a very good reason to do so (eg some fundamental misrepresentation by the landlord about he place to be rented, or some serious defect with the place) then if you have signed and agreed to pay you are under an obligatio to do so unless you can eliminate his loss by finding a replacement.