London vibrates almost as much at night as it does during the day.
There are many expat meeting spots in London. All Star Lanes in Bloomsbury (http://www.allstarlanes.co.uk) has an American theme but locals and expats alike are more than welcome! If you want to meet other expats check online for expat groups.
The Meetup site (http://www.meetup.com) is definitely a good starting point. You will find groups to ‘meetup’ for whatever interests you – whether that be dogs or tennis. If your particular interest isn’t there, start your own group and meet people.
Time Out (http://www.timeout.com/london) and Beer in the Evening (http://www.beerintheevening.com) can help you find the most happening bars, pubs, and cafes in the capital. Joining a social networking site like Facebook will also help you get in the loop when it comes to finding the hottest parties or venues where you might meet like-minded people.