Pub.

Practies it everywhere you go to. When you go for example to the resturant, ask the waiter to get your order and try to talk with him/her about anything for a while. Then, let him/her to go to see other customer. 

Enjoy

Speak to some locals ;)

Look on meetup.com for some groups to go along to there are many in London. Also look omn the gumtree website there are also ones advertised there.