As a Londoner, I am qualified to give you some proper advice:
1. Don't listen to any miserable Northerner who's brought their own cloud of despair down with them.
2. Bring your manners. They cost nothing, which will suit your Northern pockets.
3. Don't bring any of that foul, warm, flat, insipid fetid muck you pretend is 'beer'; we invented beer, and make it properly down here. It's not radioactive like Northern beverages.
4. Most folk will be only too pleased to help you out, and won't look at you incredulously, with disgust and horror, because you're not related to them. They will also be able to respond coherently and intelligibly, rather than in grunts and snarls.
5. Cast your eyes to the ground, should you encounter any of our women. You are not worthy of their attention. They won't look at you anyway, with your pallid, malnourished complexion.
An infallible list, and it'll get you by to start with.