Exhaustion! Bullying what can i do!!?

6 months ago i shared a flat with someone whom i thought was a friend. Everything was going well with the flat but i started to notice certain behavior with my flat mate that made me feel uneasy . Then i found a large amount of cash and a large bag of white powder in hidden in the communal part of the flat! I moved out straight away and all though was told that i should go to the police i thought better of it as my flat mate is a body builder and has many large friends. Recently he sent me a txt message saying that he had a large telephone bill of �200 and that he had paid it and wanted the cash from me! i asked for an itemised bill but he never produced it and continued to not only txt me for the money but had his druggie friends follow me about giving him information as to were i drank and what i did. Recently he approached me in a pub and verbally attacked me and threaten me. I paid the �200 as i felt intimidated. He then text me after i paid this money(keeping the bank receipt) and asked for another �80. I paid that across to his bank in fear he would attack me again. I went to my local the other day and he was there totally out of his head and although he did not see me i left straight away because i felt unsafe. If i go to the police and inform them of what i no And the drugs and whats happened what urances can i have that i will remain anonymous