If you are in the Uk perhaps you should find websites used by foreigners living in Britain. Russian, Polish, Chinese, African expatriate forums would be a good place to put adverts.
You need to find out where foreign communities have centres in your city. That's presuming you live in a city with enough of an immigrant population to make freelancing as a language teacher viable. Is there enough demand for language tuition where you live?
Get some leaflets printed advertising your services, credentials, etc. And have them printed in English on one side and perhaps in Russian, Arabic or Swahili on the other side. Do some letterbox drops in target districts.
Just some suggestions.