I know you've recently had loads of requests for organisations that place students with host families. If you haven't had any luck with local language schools you can try some of these organisations:
SILC - Sejours Internationaux, linguistiques et culturels - these guys are a french organisation who are always on the look out for host families in the UK. Check out their website or email delphine.v@silc.fr and she'll reply in English.
You can also try Opendoors - and east London exchange organisation
Education First - apparently the world's largest language school places foreign students and is looking for host families.
STS language schools offer courses to foreign students all over the UK and are looking for host families.
Although these look good, contacting local schools is probably the best way to go about being a host.
I'm still looking to find more organisations that organise hosting foreign students so if you know of any others that are good please let me know.