Honestly, when I came to Leeds, I was absolutely petrified about the rumours of drug culture.
I don't think I have seen a single illegal substance since arriving here and I've been here for over two years. The vast majority of my friends don't do any substances, and the very few people that do would never think to push it on to anybody who didn't want to do it (why would they, drugs are expensive). Leeds is known more for being 'druggy' than some other universities, but you need to remember that Leeds is one of the biggest universities in the country, so it's probably just more the fact that there are more people in Leeds than it being a proportionality issue. If you have issues in your halls in regards to drugs, remember that there are Residence Life Assistants (Subwardens) who will be able to assist, but I would be very surprised if this would happen.
Tldr; very unlikely you will experience drug culture if you are not looking for it, and if you do have issues, there are RLAs there to help. Drugs should genuinely be one of your lowest concerns.