I moved into this flat 5 months ago, I pay rent to the roommate which has a first hand contract. I gave 2 months notice as required in the contract. I did not pay the last month of rent as the deposit would cover it and the other roommate said I didn't break anything. She still wants me to pay but I have strong signs showing that she won't pay it back (severe drug user with debt). Today, as I came back I was locked out. What should I do ? Will the police come ?
First, I assume you mean that someone changed the lock and didn't give you a key. I also assume that you have the permission of the landlord or the local housing association for this subletting arrangement. And I also assume that the landlord gave the primary tenant permission to change the lock (and may or may not have a key, which they may or may not provide a copy of to you). In other words, there are a lot of missing important details regarding the legality of the primary tenant's action. So if this is a legal sublease and a lock-change with permission, then your question is basically, can the primary tenant forcibly or constructively evict you: the answer is that it is illegal. Eviction requires a legal process. It does not entirely matter that you broke the lease: what the primary tenant should have done is gone through the official eviction process, which would have been successful since you broke the lease by not paying the rent. Regardless of your feeling about the tenant's character, you are obligated to pay the last month's rent, and the law requires the return of the security deposit at the end of the contract.
What you should do is abide by the terms of the contract. The other guy, likewise, should not illegally lock you out.