There are generally six types of pedestrian crossings in the UK:
Pelican crossing (Pedestrian light c[a]ntrolled crossing) - these are normal crossings with traffic lights, usually at road junctions. At these crossings, you should cross when your light turns green.
Zebra crossing - these are the usual parallel white lines painted on the road, as in many countries. Usually you'll also find blinking amber bulb lights on both sides. At these crossings, drivers must stop and let pedestrian cross, so you can just cross the road (taking due care, of course).
Puffin crossing (Pedestrian user-friendly intelligent crossings). These are also light controlled, but normally also have video cameras (on older model) or infra-red sensors (on newer model) that spot approaching pedestrian ready to cross and change the traffic light red for vehicles and green for pedestrians. When the traffic light changes for vehicles to amber, they should let the pedestrian already on the crossing pass, but can go otherwise. For pedestrian, when the green light starts blinking, you should not start to cross the road and wait for the next green instead.
Toucan crossings (from Two can cross - pedestrians and bicyclists). These are, again, light controlled, but normally display red to pedestrians and green to vehicles. To cross, the pedestrian would press a button on the control panel near a crossing and wait for the light to change. Many newer crossings also have a time display showing how many seconds are remaining before the light changes to red again.
School crossings - these are found just outside schools and are usually operational during school hours. Most often you will find a lollipop lady (or gentleman) - a live person with the STOP sign on a long stick - who would periodically step into the road and show the STOP sign to vehicles, who must stop. She/he would then allow pedestrian (usually, lots of children going to or from school) to cross.
Outside race courses and in some rural areas (especially in Scotland) where there may be many horses, you can also find a Pegasus crossing. These are similar to Toucan and Puffin crossings, but have special provisions for horse-back riders (e.g. a second control button higher up the pole and/or a second sensor pointing at higher up).
If you happen to be on a street where there are no pedestrian crossings, then you are free to cross taking the normal precautions (i.e. make sure there are no approaching traffic). Be mindful of occasional signs "No crossings" directed at pedestrian. While I haven't myself heard of anyone being fined for J-walking in London, I'm fairly certain that there's a section in the Road Traffic Act that makes it an offence.
So, to summarise, try to use a pedestrian crossing if you can find one - or use your common sense and judgement.
