Maybe you should suggest a night out, for you and the people you say you enjoy brief conversations with. Choose somewhere closer to your home so you don't have the restrictions of getting back. Also, relax those restrictions a little - i often travel an hour away from home on a night out and make my way back at 2:00am (as you are in London there are sooo many night buses - all of which i have ever been on are quite full even at that time). Instead of waiting for them to invite you to their lunches etc. perhaps take the initiative and suggest lunch/coffee/cinema. It must be hard living at home as the making friends process is more delayed than everyone else...
As for societies, don't let the confidence decrease stop you from going to events (even if it is alone) as this will make it even lower in the long term. The next time you think 'no i can't' to an event, make yourself go, you might surprise yourself. Or, and this is my final suggestion, i promise, you could find out about other people on your course/other courses that are living at home. These people will possibly be feeling as you are, you just need to find eachother! lol. I'm quite a shy person and terribly unassertive but recently i've told myself to just go for it - it really does work. Open your body language and people tend to respond, suggest meetings and people tend to accept - or failing the subtleties, speak to the few people you mentioned about how you are feeling, maybe they are unaware that you feel isolated?