Yamasa has several short-term programmes like their "Acceleration Program" or "AIJP". You can even use a tourist visa to study in the three-months programmes and save yourself the effort of having to obtain a student visa.
Their website is a mess. Have patience and read through all of it. You should find what you are looking for. You could also E-Mail them. They are really nice.
I initially wanted to study Japanese at Yamasa for two years, but since that is super expensive, I decided that it may be smarter to study Japanese at a university in England and do a year abroad. But if no unis accept me, Yamasa is still my backup plan, since their applications only open end of March. :3
Not exactly, but I can give you some resources:
http://jls-guide.com/english/school_list/index.html (a directory of Japanese language schools)
http://www.jref.com/forum/forumdisplay.php?f=181 (decent forum about studying in Japan)
Also, a Google search for "<name of school> sucks" can often help to find anything negative about it.
Some things to consider:
Do you have the necessary dedication?
What exactly do you want to learn? Just speaking? Reading and writing?
Would you mind if 8 of 10 in your class are Chinese or Korean who don't speak English?
Are you okay with living alone thousands of miles away from home in a country whose language you don't speak and whose customs you don't know?