I suggest you find out where various communities hang out. You find in most large cities that each foreign community has a place where they meet. It might be a café, restaurant, food shop, book shop. These places become the hub of a community and there are often notice boards where you can advertise for free. Also put ads up in libraries, and contact consulates to find out if they have libraries. Find out about foreign press, I know that there is a free magazine called Ici Londres for French speaking people in London; find out if other communities have similar magazines or newsletters. Don’t forget churches, mosques and other places of worship; you might be able to put an ad up on a notice board, in a newsletter or in a meeting place nearby. Then of course, the usual websites which you probably know better than I do.
I can’t be more specific because I have never worked in London. Hope these ideas help.