London is a very big international city.
As with other very big international cities, there is a fair bit of crime, the majority of which occurs at night. It's also mostly concentrated in central london and the rougher parts of london. The rougher parts are pretty much everything in Zone 2 give or take. Some areas are more notorious than others, hackney, brixton, mile end, stepney green, shadwell, clapham get a mention.
If you are in a large enough group, on a well lit road, you should be "relatively" safe. Though find yourself in one of those rough areas I listed before, they probably wouldn't mind having a go at mugging the group.
Do not ever walk alone at night, you are asking for trouble. The only people out at night are the drunk clubbers, prostitutes, police and criminals. You'd rather not meet the last bunch.
If you can avoid going out at night, great!! But if you have to go out, be very careful where you go and who you talk to.