How did it go? Have you had to get it done yet? The man sounds thoroughly unpleasant and you're obviously pretty nervous about him. You will get out of this situation and it will be a massive relief when you do. I'd use the environmental conditions angle too, like Richter said...it's affecting your respiration and general health. You can even try and flatter him, thank him for being so understanding before he has time to comment, say you struggled with how to tell him because you didn't want to disappoint and apologise that you have to leave. Basically lay it on really thick,.
You have to do something to get out of this stale mate and better your health in an environment you deserve without this presence looming over you. Your T and GP can help. You have an agreement that he signed however flimsy it may seem, it's a signed document. After April 2009 in became UK law that ANY landlord taking security payment did not have an automatic right to keep it.
If he gets shirty, well I'm sure there are environmental health issues that he is probably infringing by letting his property, which does make it a business an he s taking money of you. Do you pay directly into his bank account.
I know this is probably nerve wracking but once you get it done it's only going to get better.