[Sangetsu]

You are not allowed to work in Japan with a visitor/tourist visa, period. Once you get your spouse visa (which is not automatic, your spouse will have to prove that they earn enough money to support the two of you), then getting a job is easy.

If you are in Japan on assignment from another country, then you aren't technically working in Japan. You will need to open a bank account in Japan to get paid here, and you cannot open an account with a tourist visa. If you are getting paid into a UK account and are simply withdrawing money from an ATM in Japan, you will be okay in the short term, but anything over 200,000 yen per month withdrawn in Japan may be taxable, especially if you change to a spouse visa.