What to do in London at night time?

Hey. So I'm taking my sister to a concert in Soho next week. We will arrive in London in the morning and I'll spend most of the day looking around and shopping but her concert starts at 20:00 and ends at about 23:00 - midnight. Shops around this area (Oxford Street, etc) close at around 20:00 so I was wondering if there was anything I could do during that time until my sister gets out of her concert. I haven;t been to London much before so I don't know the city well. Thank you.