Student Visa

'A student visa will let you stay in Japan for a period from 3 months, to 4 years and 3 months. '
'The maximum amount of time someone can study at a language school is 2 years.'

My plan is to study both Japanese and another subject in the UK to the 'required level', then study Japanese in Japan for the maximum 2 years, then the second subject for 2 years after that so I can stay for around 4 years total. I will also have enough time to get a decent amount of money saved for that side of the application. Pretty simple questions, what level of qualifications do I need to then be able to furhter study in Japan, and is there anything wrong with the plan?

Questions about the required qualifications to enter a study program must be asked to the relevant institution.

Have you checked about studying this mysterious "second subject" in both the UK and Japan? Do you know if there are programs at UK and Japanese universities allowing you to get credit from the other country?

Sounds like you're still looking for a "quick route" into Japan without doing any real research. You will need to put some real effort into this, not just look at the visa conditions.

Do you actually want to study? Do you want to study Japanese? Are you going to be happy studying full-time, attending all classes, etc. to keep your student visa valid?