I drove without lessons. I'm gonna take the test as soon as I can get a proper job and get some money.
My best suggestion is to approach this as positively as possible. Yes, things are different but the more you harp on it, the harder it is to make the adjustment. If you can afford it, get the guide to road signs and traffic rules from WHSmiths. The library may have it, too. This way, you can begin to familiarize yourself with signs and rules even if you can't afford lessons. If you've been driving for 12 years in the US, there's no reason why you can't drive well here. Yes, there are roundabouts and narrow roads. But they aren't impossible otherwise you wouldn't see all these cars on the road!
Have you driven a manual before?